Galan007
|Quantum Observer|
I have a theory that because Saiyans are purely a warrior race, they pass their power-related traits onto their offspring when they breed.
IOW, if Vegeta/Goku were capable of reaching SSJ when Trunks/Goten were conceived, perhaps the ability to unlock the form became that much easier for the boys... Almost like it was part of their natural growth process. That would explain why Goten, who had extremely limited ki-control(he couldn't even fly, ffs), could still access SSJ. Additionally, both Goten and Trunks were already full-power Super Saiyans(as evident by their calm/playful/relaxed demeanors), despite having very little training with their SSJ states.
So yeah, things like that just make it seem like accessing SSJ had become part of their natural growth cycle--akin to puberty, except much earlier. 🙂
...There's also the fact that Saiyan/human breeding begets very powerful offspring(as noted by Vegeta)--so they'd already have one-up on purebred Saiyans.