He needed a Berry Gordy to properly package him and present him to the world.
Berry was very weird about bringing kids aboard unless they were Little Stevie Wonder level talent. Stevie was like 11 when he got signed and they had to make all kinds of arrangements for him + get him a tutor. But aside from that, he was a worker.
When the Jackson 5 pulled up their act was a little more polished and a couple of the brothers were "older," but they had hits out the gate so you can't argue with success even if it was from a kiddy group. In the very least it meant Berry could get like 10 years of work out of them and put them around the best producers/writers in black music.