I wish we could.

The problem is, individuals have to be a certain age to legally give us consent to store their PII (personally identifying information). The age where kids can do this varies (US:13 / EU:16 though not all countries have updated local laws which creates confusion). If you aren't that age, we can't keep your account, because we're not allowed to have the information necessary to establish one.

If we can't keep an account, we can't keep any sense of normalized ownership of posts and other contributions. We can't let them log in because that means storing a credential which contains, yep, PII.

I had to delete my daughter's account from Arqade when she started answering Minecraft questions (quite well, in fact) because she wasn't 13 yet and a US citizen. I completely feel for this, but the law is what it is.

I wish they could. But:

The Children's Online Privacy Protection Act requires that websites cannot collect personal information from children under the age of 13, including "full name, home address, email address, telephone number or any other information that would allow someone to identify or contact the child."

This also goes for users in the EU that are under 16:

The update will reflect that you need to be 13 years old, unless you live in the EU, which means we require you to be 16. However, there's nothing in the law that says we have to go proactively looking for underage users, which means the policy isn't any different from what we have, essentially. If someone says "I'm only x years old" and mods suspect the user might be underage, they just escalate it to us to investigate. A lack of action on our part is an indication that we looked, and found no grounds to delete the account.

Basically SE is not allowed to keep any user data (including the e-mail address you use for signing up and registering your account), and so a suspension isn't feasible. Moderators escalate these underage users to Community Managers instead of deleting them ourselves, to ensure all information is properly scrubbed.

There is a legal reason for deleting their accounts in full, rather than simply suspending them.

In the United States, there's a law called the Children's Online Privacy Protection Act (COPPA) that requires any website accessible in the U.S. to not collect information from any user who is under 13, and to erase any information that has been collected from users who are under 13.

As per the relevant FAQ, How do I use Stack Exchange if I'm under 13 years old?:

The Children's Online Privacy Protection Act requires that websites cannot collect personal information from children under the age of 13, including "full name, home address, email address, telephone number or any other information that would allow someone to identify or contact the child."

Since Stack Exchange, during the registration process, allows users to enter their name, email address, and website URLs where other users might be able to contact the user, Stack Exchange is legally obligated to follow this law.

[...]

I'm under 13 and I created an account. What will happen?

Your account will be deleted and all information you provided will be removed from our servers. You can create a new account once you turn 13, or follow the procedures above to have your parent/guardian create an account. While you will be able to participate in Stack Exchange again, unfortunately, we cannot reinstate your old account, so any reputation you gained on your old account will be gone.

Since all their information has to be removed, the account has to be deleted in full, and the details have to be purged so that even moderators can't see them.

Note that in the past, SE used to suspend underage users until they were old enough to participate, as you mention:

As you know, users that are under the age of 13 can't participate on our sites, as per our terms of service. The way that we used to handle a disclosure from someone that they were only ten years old was to suspend them until they were old enough to participate. That required us to be able to suspend someone for longer than a year, so the maximum time the system would accept when a suspension was issued used to be quite high.

We changed that, moderators are no longer able to suspend accounts for more than 365 days. The community team now deals with reports of users under the age of 13, and we handle it quite differently.

I guess their legal department informed them that this would not be in compliance with COPPA, as it requires all information to be purged.

A quick note: Crucially, the law does not require that SE actively go out and seek underage users, or verify age; it simply requires them to take action if they happen to know that the user is underage (e.g. due to a self-disclosure, a report, etc.).

Collecting personal data without age verification or parental consent for users under 13 may not be COPPA compliant for Stack Exchange anymore

COPPA (the Children's Online Privacy Protection Act) is the US law governing websites' interactions with children under 13. Stack Exchange currently uses an exemption in the law that allows sites that are general-purpose or have adult-focused content sites to largely ignore COPPA, only being required to delete any personally identifying information from any user that they learn is under 13 years old. Such sites are not required to proactively take any steps to determine that users are 13 or older, only react if someone notifies an employee.

That exemption applied when SE focused on technology professionals, but it has since expanded in scope. It only takes an identifiable part of a website that is attractive to children to make the entire site fall under stricter provisions under COPPA. While a generic chat site would qualify for the exemption described above, a chat site with several rooms, one of which discussed dolls would not qualify. That doesn't necessarily mean the entire site is subject to onerous rules -- it depends on the nature of the site and which exemptions the operators are able to take advantage of. COPPA is an extremely intricate set of regulations, so the correct answer to almost any question about it is "it depends."

SE now clearly hosts content that is attractive to children. For example, at the time of writing, the most popular tag on Arqade is minecraft, and 20% of Minecraft users are under the age of 15. This means in the lingo of COPPA, SE is considered to be a website that is "directed to children" but "does not target children as its primary audience."

While websites that are directed to children but do not target children as their primary audience do not qualify for the exemption allowing them to collect whatever PII they want and delete the PII of any underage user they happen to find out about, they do qualify for the active age screening exemption. This allows them to ask for the user's age before asking for PII, which allows websites more flexibility in how they treat people above the minimum age. Websites in this category are not required ask any user their age, but the default flips; they are required to treat all of their users as children that they don't know to be adults.

The flexibility for sites in this category to treat adults and children differently is not unlimited: according to the FTC's official COPPA FAQ, they cannot prohibit children under 13 from using the site:

4. I run a site that I believe may fall within the FTC’s sub-category of a website directed to children but where it is acceptable to age-screen users. Can I age-screen and completely block users who identify as being under age 13 from participating in any aspect of my site?

No. If your site falls within the definition of a “Web site or online service directed to children” as set forth in paragraph (1) of 16 C.F.R. § 312.2, then you may not block children from participating altogether, even if you do not intend children to be your primary target audience. Instead, what the amended Rule now permits you to do is to use an age screen in order to differentiate between your child and non-child users. You may decide to offer different activities, or functions, to your users depending upon age, but you may not altogether prohibit children from participating in a child-directed site or service.

This rule was made under authority from Congress to prohibit deceptive practices, so it appears that the FTC considers a site that has content that is interesting to children but children are not allowed to participate in to be deceptive.

SE's options if they want to keep Arqade and any other sites that are interesting to children are varied and complicated, but the two most important things to keep in mind are that they must not prevent a child from participating in an activity because they haven't provided PII that is not reasonably necessary for the activity 16 CFR 312.7, and they must obtain parental permission if they collect/use PII for any reason not in the fairly generous list of exceptions.

After digging through the ins and outs of what is considered PII for COPPA purposes, I'm convinced that pinging and chat are the only major pieces of functionality that require PII currently¹. That doesn't mean there aren't other issues that would need to be addressed. For instance, SE would have to ensure that the tracking used on the site by itself and its advertisers apply, and it would have to take steps to make reasonably sure that posts and comments by underaged users does not contain PII. (Pre-screening for PII by a staff member before publishing is explicitly defined as a reasonable approach for doing this.)

Alternatively, SE could obtain parental permission to display any personal information children put in post or comments. Third party services exist to make it easier to verify parental consent. They would still need to make sure they were in compliance with the various provisions of COPPA, but with parental permission there is much less that is restricted.

1: But note that the presence of pinging and chat turn anything that displays a username into "requiring PII" due to odd dependencies² in the COPPA definition of PII.

2: This is not even the oddest interaction³ between definitions in the COPPA regulations.

3: I really want to see a ruling on how Chevron deference applies to an agency's resolution of Russel's paradox!