it all starts at 8U
There is one rec league in my area, starts at 10U. Before that, they play tee ball and unless the girl is an obvious super athlete, they just get shuffled to the outfield and ignored while the boys get all the attention.
It's no surprise that many girls get bored and quit after a year or two and never find their way to softball (not that girls playing baseball is bad, but they often don't stick with that, either).
It really is all about opportunity for them. We're starting a 10U team at the school this year. It's incredible how much interest there is from girls that age. But for many, if they don't play when young, they think they can't so they'll never take the chance when they get older.