...for me part of what makes him creepy is that he snogs everyone (OK - with Lilah he was, strictly speaking, someone else).

There is an actual age difference of about six years between the actors, if I remember rightly. Although these days it looks a lot better as SMG seems to have aged disproportionately over the series. I found her far more believable as 15/16 in season 1 than I do as 21/22 in season 7.