Young women are sexually attractive for the reasons you state, and I do think that some older women resent the attention that gets, but there's more to it.

The difference in maturity means it's harder to relate. I personally wouldn't want a young girlfriend, despite finding them sexually attractive, because I could not imagine trying to share my life with them - our interests would be too divergent. I think there's a real risk, when one party in a relationship is much less mature, the other party may have an unbalanced or unhealthy set of interests - interests that may end up hurting or harming the less mature party.