The accepted answer is very misleading.
The first two parts of §9 of the Jugendschutzgesetz apply to pubs, shops, and other public places.
In these places, it is illegal to sell beer, wine, and similar drinks to children and to adolescents below 16. It is also illegal to allow them to consume such drinks. However, this doesn't count for adolescents accomponied by a guardian, it always applies to children. Note that a child is someone under 14, and an adolescent someone who is not a child, but under 18.
It is also illegal to sell other kinds of alcohol to any child or adolescent or allow them to consume it, even above 16, and even when accompanied by a guardian. This is mostly about hard spirits, but also about "alcopops" which are regarded especially dangerous to children because the alcohol is hidden, even if the alcohol contents is typically only about the same as in beer and much less than in wine.
(Also, vending machines selling alcohol must not be accessible by children or adolescents, that's part 3).
This doesn't say anything at all about handing alcohol to adolescents and children in private, and allowing them to drink, nor about children and adolescents finding and consuming alcohol at home, nor about buying alcohol in the presence of children.
At https://www.t-online.de/leben/familie/schulkind-und-jugendliche/id_49295364/jugendschutzgesetz-2018-alkohol-ab-wann-und-was-ist-erlaubt-.html someone makes the claim that during private parties (at someone's home when other children or adolescents are invited) parents would be responsible to obey the same laws - whether this just someone's opinion or the law is impossible to say.
And that is what Google will show you: I cannot find one article that states whether allowing children or adolescents to drink in their home in private is either legal or illegal. There are many articles that claim it is bad, and parents shouldn't do it, many that refer to §9 which doesn't say anything about private alcohol consumption, but not one making an explicit statement either way. It would be reasonable to assume that harming your children is illegal, but if there is no proven harm, it seems to be legal.
As to the question: Alcohol consumption can cause damage to anyone, any age, but doesn't necessarily cause damage. Causing damage to children or adolescents is obviously illegal, (just like causing damage to adults), so giving alcohol to them in amounts that does cause damage is illegal. The specific laws that I could find have been linked to, but I couldn't find anything about alcohol in a private setting.