The success of this unioin will depend on the maturity of the gal and your willingness to accomodate her.
The oldest this gal could be is 23 and while I find this too youngish to settle, there are some ladies who at this age are done with younger men and want marriage et al.
I have seen a cases of significant age difference going well and many cases of same age marriages going horribly wrong so don't sweat the little stuff.
"The purpose of bureaucracy is to compensate for incompetence and lack of discipline." James Collins