You need at least to be six year so you can drink it?

So I just looked at the wiki article and it said it had very lower alcoholic content, but it's still fine. Maybe read that.

You need to be at least 17 (of age) to drink firewhiskey, just like with alcoholic beverages in the muggle world

I just looked it up- most soda's have a very low alcoholic content as well, so it's very similar to a soda

It can only intoxicate House-elves.

Wait, sodas have alcohol? Never knew that

Like 0.05% or something like that

It’s not alcoholic I believe

It's not alcoholic it has something like 0.05% and toothpaste has more alcohol than it (0.63%)

Butterbeer isn't wine, it's practically a sweet, warm, caramell like, carbonated drink. A tiny bit of alcohol, like @PuppyPotter27 said, but nothing that would harm 13 year olds.