I like to think of the saiyan gene like a dominant trait. Even if they mix with humans, the saiyan trait can still be as potent as a full blooded saiyan. Humans have stronger emotions, so they can turn super saiyan earlier.
But Gohan got stupid weak in his high school years, to the point that he couldn't even hit SS2 again. Akira Toriyama was really absent minded until Super where Gohan got his dad to SS Blue just to beat him.
I like to think of the saiyan gene like a dominant trait. Even if they mix with humans, the saiyan trait can still be as potent as a full blooded saiyan. Humans have stronger emotions, so they can turn super saiyan earlier.
But Gohan got stupid weak in his high school years, to the point that he couldn't even hit SS2 again. Akira Toriyama was really absent minded until Super where Gohan got his dad to SS Blue just to beat him.