The plot needed them to be. Gohan is more impressive than them though
This
Toriyama wanted Trunks and Goten to replace Goku and Vegeta, but needed them to be strong enough to actually contribute. So he gave them SSJ, and never explained how they got it.
The theory that Goten and Trunks are so strong for their age because Goku/Vegeta were Super Saiyans when Goten/Trunks were conceived has one major flaw in it. Future Trunks. We know that Little Trunks is stronger than Future Trunks was at his age, even though they should be similar, if not F-Trunks being stronger due to the Androids threat.
This too.
Future Trunks and present Kid Trunks were conceived at the same time and born on the same day. Yet there is a huge discrepency in their power levels.
Future Trunks became a SSJ at 14, after years of training with Gohan and fighting the androids.
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At 17, Trunks neg diffed Frieza and King Cold, but was shown to be beneath Post-Yardrat Goku
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[video=youtube;ikPh3xXhx7c]https://www.youtube.com/watch?v=ikPh3xXhx7c[/video]
Present Kid Trunks got SSJ at age 8, with very little training or supervision from Vegeta. Also a "supressed" charged ki blast was enough to making #18 start sweating.
So yeah, Trunks and Goten are powered by plot. It's hilarious, because on DB Super and the movies that lead up to it, Goten and Trunks' power levels have either decreased or been scaled back by the writers. To the point that they are stated to be around 1st form Frieza level.