Sounds like you just met the guy for a few hours before you two had sex..
Man: Let's have sex.
You: Wait wait! How old are you?
Man: 23
You: Aight, let's fuck.
Frankly, if you don't care for knowing a person before you're going to have sex with them, why care about if he's in it for teenage sex or not.
Also, I remember talking to an "import" curry ages ago.. he told me that his passport age and his real age is different.. and the passport age is younger for more employment opportunities.. so the guy you're fucking is possibly 30yrs old. I think you should be able to tell the difference between a 23yr old and someone who is or near approaching 30yrs old.
From what I think, he probably has a billion girlfriends and he can't remember how old he told you he was [cos he tells different girls different things at the same time] and thinks of a random number between 20-30.