I don’t think his age is mentioned so it could be anything. But I disagree that he is necessarily a kid just because of his looks, I see plenty of adult uke with the same physique and usually no one is schocked. And even without the age mentioned they would believe they are adult... But since Eve is the top in this case, its looks unusual and make people confused.
Eve is surely older than 15, my guess is 18-19 at least, since at the beginning we see gozu holding a high school entrance exam book, which means he was around 15 despite being drawn much younger, I'd say drawn like a 10-12 year old. So if the artist draws the characters at least 3 years younger than they're meant to be, I'd say eve is a solid 18.
I once commented with a long ass paragraph because I started to notice some details. I also predicted makaddies introduction! Makaddies has 2 other friends as well, one of them is a girl, the other one has a lord farquad lookin ass haircut. Their names are kirikiri and ouie(? The writing was really small and hard to read)

Woah he got such top energy rn.....
ლ(´ڡ`ლ)