On other sites i have been contacted by younger ladies, 'girls', some as young as 15. Anyone under 20 i have not replied to. Just an arbitary cut-off i feel comfortable with.
Some have just wanted to chat and make friends. Some to be online g/f and/or cyber groupie, whilst a couple have wanted an older guy for a full on relationship.
I have found, to my surprise before joining dating sites, that many younger woman have fantasies about love or relationships with older guys. For most though that is all it is, a fantasy. For most it should stay just that.
I had a relationship with a 21 yr old from a different site. She left me for a younger guy. Not sure if there is a moral in that.