Those two are naturally related. If getting pregnant at 10 usually lead to death, then your genes will avoid that if getting pregnant at 10 is common.

However if socially we prevent girls from getting pregnant at 10 then biology can make them go through puberty at 10 without risk, since they will not get pregnant anyway.

Edit: As for the playfulness point, for us humans we punish kids for being playful when they reach 7, so it makes sense that we will evolve to stop playing around at 7 and hence becoming adults already by then. We don't punish pets for being playful though, at least not today.

I'm finding it hard to understand your point or not finding that argument particularly convincing. We can socially change much faster than we can biologically evolve. Meaning, the change in birth age can be heavily influenced by social factors (birth control, resource abundance, requirements for self-sustainment in adulthood) much faster than biology would adapt. We can be extremely socially different than those who lived when it was the norm to be pregnant at 14 while being essentially biologically identical.

Further, we can have multi-modal distributions that undermine the biological assumption. Some groups show much lower birth ages because it's not culturally disadvantageous than other groups. In that context, even if the average age increases, it doesn't mean lower birth age is biologically disadvantageous.

>humans we punish kids for being playful when they reach 7

This needs a citation or at least a definition of what is meant by "playfulness", because I find it hard to believe. I don't know any parents who would want their child to stop playing at 7 years old.

> I don't know any parents who would want their child to stop playing at 7 years old.

Parents don't decide what happens to the kids at school though.

> We can socially change much faster than we can biologically evolve.

If the genes were always there but they died constantly 200 years ago it just meant that we no longer cull those people early, and after a couple of such generations we now have them around. Early puberty is still not common, it just exists, that makes it more likely to be biological than social, if the social changes did it then early puberty would be the norm today rather than a very rare exception.