Of course, legally you can do what you want, but please keep in mind that there's usually a big difference in maturity level between an 18-year-old and a 25-year-old.  You might find that in a couple of years you've changed and you don't want the same things you wanted before.  Of course, the person you feel this way about will probably change in that time too.  So choose carefully, knowing that both of you will be growing up...\n\nDon't do the sex before marriage thing, please.