Wasn’t puberty later back then too? Like people weren’t waiting around post puberty saving themselves for whatever, puberty just happened alongside full adult body maturation, not before as often happens today.

Puberty even today has to do somewhat with body weight. You have to reach a certain level to get it. Malnutrition may have delayed it in prehistoric times.

actually the later puberty ages may have been a temporary side-effect of malnutrition common during industrialization, there's some evidence that hunter-gatherers (and even people during medieval times) had good access to animal protein, fats, and other necessary vitamins and minerals from plant life (nutrition plays a big role in puberty onset)