So, I'm glad this was brought up. They do not have to be teens. What distinguishes them from the League is all of them are still in the early stages of their hero career. So, having them be around 30 is fine, but for it to work, they weren't superheroes for most of their early life. That's not say they re powers didn't manifest at an early age. Ideally, your character has been at it for less than 5 years, as they we're not active during the events of the first titans. That is not required tho. I don't want  hinder anyone's creative process, so if you have a different idea don't hesitate to throw it out there.
Tl;Dr Inexperience is what separates the Titans from the League.