No, i mean from a maturity angle. Both people have too much growing to do before they could get together.
Legally, by the end of season 7 they were at a stage to pursue a different relationship....but neither were ready for each other.
I suggest 23/24 as that gives Dawn the time to finish school, understand who she is and what she has to offer the world a little bit more, and it give Xander time to truly define himself career wise, and be able to work at a relationship properly with Dawn...plus the baggage of Anya is gone by then
I'm working on a basis of the character we actually saw, not what could have happened back then.