Was there ever a reason for why becoming a super saiyan became easy for the trunks and goten? Related to this, but I would like to know specifically for (young, not future) trunks and goten, how they managed to become super saiyans at such a young age - the others all went through a stressful emotional event that pushed them over the edge, yet while these two were growing up, I can't imagine them going through anything similar - there was no villain at the time.
The mixture of races is the answer. Gohan also become very young, but had more difficulties to release its power.

In the anime it is said that Goten and Trunks had fighting each other for fun, it probably gave them the skills needed to become.