I’ve mentioned this in passing, but I think it’s worth establishing that Steven seems to have stopped developing normally when he moved in with the Gems—all-but immortal beings who never themselves change without outside influence and have little understanding of humanity.
We can see this in the extended introduction:
At the start, he’s visibly younger: smaller, higher voice. His clothes are ill-fitting. There are gaps from his baby teeth. It’s implied he’s only begun to learn the uke, though he’s picked it up quickly.
(Mind you, it’s on record that he stopped aging at about eight, which in typical human terms, for what that may matter, lines up with the adult teeth growing in.)
The house is still being built, but it’s pretty far along. Fast forward through a building montage; next we see Steven, which it’s reasonable to count in months rather than multiple years, he’s moving into the newly-completed house. Here, he looks the same as he does in the show.
After he leaves the care of Greg, the Gems don’t engage with Steven. They hide everything. Garnet waits for his back to turn, to retrieve and bubble gemstones. Until he forces their hand (mostly after meeting Connie!), they continue to treat him as he was when he moved in.
And so due to his strange hybrid physiology, where like a Gem his physical form corresponds to his self-concept, Steven never progresses past the mentality of an eight-year-old—ergo, for something like four years (again, until meeting Connie!) he never grows up.
Without her, Steven may never have started questioning the miraculous life around him that never quite adds up, never pushed back against the Gems and started to claim his own agency… and the Earth would very probably have been destroyed about a year and a half from “Gem Glow.”