I have had students take the Private checkride at 42 hours and up to 60 hours. Solo happens when the solo is ready to happen, the student better be able to do EVERYTHING involved in the traffic pattern by himself. No touching the controls or talking on the radio from you.

Every student is different and needs special consideration, for example I currently have a 78 year old primary student. At 78 years old that would give him the hand eye coordination of a (drumroll please) 78 year old. He is progessing quite nicely but a 20 something will have a easier time handleing the controls.

I have seen 900 hour student pilots, but they are in the middle of nowhere with the closest CFI sometimes being a hundred miles away, these guys just never took their checkrides and bought their own planes. There are some that trained to solo and some that taught themselves. Scary.