No no, it increased their attack and defense stats, which determines how much they hit/dodge:
Source: frostbeastegg guide
The biggest Bannerlord apologist strikes once again.
I don't think it has to do with the low level requirements, but the low level+focus point xp multiplier.
Back in 1.8 and 1.7 I couldn't choose a companion as heir, only wife and daughter.
I just created a new save (sandbox) and hired some companions, and in a few days they started to spawn. I think the game tries to maintain a total of 32 unhired companions.
Pretty sure you get a "Game Over" screen, as when the only members in an AI clan are underage kids the clan disappears.
Pretty sure is campaign exclusive, part of the main quest