define "something."
if we're talking random strings-free encounter, all that's relevant is whether or not she's legal and you can live with the taunting of your peers if anyone finds out.
if we're talking about deep meaningful blah, blah, blah, then for me the cutoff would be like 30. (i'm older than)
but that's really about where people tend to be in the whole self discovery thing at various stages of life, and it's specific to the people involved.
there.
that's my overly nuanced response to your simple question of how old would a girl have to be for me to bang her.