I don't think it's abnormal, it's very common indeed. Perhaps less so if you're exclusively or particularly attracted to older women, but even then it's not a problem. As for the reason.. could be you are subconsciously seeking mothering/care-taking, either because of feeling emotionally immature and vulnerable, or perhaps because your own past experiences of being mothered were inadequate (or even over-bearing). If you have trouble hitting it off with girls your age, it could be as simple as finding that older women are more responsive and forgiving towards you, which reinforces the attraction. It could be more a sexual thing - as with a fetish, some kind of past experience or association which has caused your mind to equate older women with arousal.