Absolutely old enough to make their own choice.
I'm vegan. I have a 14mo old. We make vegan meals at home, because I want her to eat what we're having, and don't want to make two separate things. I do however buy her in things like cheese and eggs, because I want her to carry on being exposed to the allergens, and I don't want her to never have dairy then struggle to digest it if she does want to eat it one day.
She has both meat and dairy when at my parents/nursery.
As she grows up, I will expect her to eat what I'm making, but would buy her whatever when out. If she's a teenager and wants to spend some pocket money on chicken dippers or whatever, fine. I will always be happy to buy in dairy, etc, as that can be added to cooking without making a completely separate dish.
I would never stop her eating cake etc at a party.
If people think I'm a bad vegan then 🤷🤷 I think people take it too far and I don't think it's right to impose a diet on anyone.
On another note though, if a kid isn't used to dairy, I'd be reluctant give it them if you're not the parent, as it's likely to cause a very upset tummy.