18 year old and 25 year old...?

Age is nothing but a number... but considering she is so young that makes her more vulnerable than someone with more life experience. Respect her always and remain there for her even if things don't work out. Something else to think about... it sounds like the age gap may be bothering you or you're worried what others will think and at the end of the day you are the one living in those shoes so don't let another's personal opinions bother you.