He became a super sayan during his battle with Freeza, after Gohan was born and was like 8, so I'd say about mid to late twenties.
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Gohan turns super saiyan in episode 145 while training with his father Goku in the hyperbolic time chamber. Goku was blasting Gohan with a Kamehameha wave and while Gohan was blocking it he transformed into a super saiyan! To add to this, Gohan thinks/remembers all the times when he let his friends down, then while blocking the attack he starts to get upset/angry with himself for letting them down, then he turns super sayan for the first time, but he can calm down so Goku tells him what to do then he is in normal form again and on his knees because he is tired from transforming. I hope it helped.
When he was in the Hyperbolic Time Chamber training to beat Cell. I think he became Super Saiyan 2 after he died to save the world from Cell. If Goku could turn Super Saiyan 2 he would have beaten Cell himself, instead of making ohan fight him.
Yes, Goku wins the battle and leaves in time but the people on earth try to wish him back to earth but he decides to train in space as a super sayan than come home with his family. But, Frieza is still alive and with his dad king Cooler to give revenge to son Goku but Goku is still not on earth yet and might come home to the horror on his friends death.
Gohan is stronger if goku is in his normal form but if they are both in super saiyan 2 form or if Goku is in super saiyan form then goku is stronger.
goku turns into super saiyen after freeiza full power killed krillen. goku needs to get angry to turn into super saiyen