Can つ be used as a counter for ages 1 to 9?

Question

歳 seems to be commonly used as the age counter, but I noticed that this article (see section on ages 1-9) mentions that つ can be used as an age counter:

来年で５つになるんです。

instead of maybe something like

来年で５歳になるんです。

This Wikipedia page says

��齢: 漢語の「歳」を使うほか、1から10までは「つ」と同様に言うことができる。

which I read to mean "つ can be used the same way as 歳 for ages 1 to 10" so it supports the idea, but the article cites one reference for its entirety so I don't know how trustworthy it is. I also searched goo.ne.jp which says

［接尾］助数詞。和語の数詞に付いて、物の数や年齢などを数えるのに用いる。

so it can be used for ages (unless it refers to age differences), but it doesn't constrain it to a range. My questions here are

1. If つ can be used as an age counter, is it only 1 to 9 or beyond as well?

2. If used as an age counter, is it considered more informal over 歳?

Answer 1

Describing one's age using つ is very common. If you ask a 3-year-old toddler "How old are you?", you will hear either みっつ or さんさい. I don't know which is more common. Since つ is only for 1 to 9, つ for describing age is also only for 1- to 9-year-old.

つ tends to sound more friendly. It's perfectly safe to use it in formal or friendly conversations with your boss, but 歳 is preferred in academic or medical contexts. (Well, in academic contexts, つ is not preferred even as a generic counter. People usually choose 個.)