This is something I actually have experience in, I was briefly with a girl who just turned 18, and I was a few months shy of 24, I would say thats its okay, other people may not always see it the same way though.
I had a friend who thought that it was a bit too much of an age difference, I asked him why it was okay for him to be getting married to a woman 7 years older than him, that shut him up pretty fast.
Anyway, all that matters really is that you 2 are fine with it, I ended up being a little uncomfortable with it so I ended the relationship...then a few months later I got in a massive car wreck and nearly died...it was a bad year for me.