It is not. See JuSchG § 9, section 2: "Section 1, number 1 does not apply to adolescents in company of a legal guardian". Section 1, number 1 restricts beer, wine, cider and similar drinks under the age of 16 and an adolescent ("Jugendlicher") is defined as 14+.

In total, that means that you are allowed to buy and drink the listed kinds of alcoholic beverages starting at 14, as long as a legal guardian is present and allows it. Even in public.

For other alcoholic beverages (e.g. liquor) or food that contains significant amounts of alcohol, there is a hard limit at 18+, no exceptions.