The regenerative powers of an adolescent body may allow a body to recover faster, but they have to survive the pregnancy. The human female body is not at full size until after the 18th year.
And there is the psychological maturity, which doesn’t happen until later. The brain is not physically mature until the early 20’s. The psychological maturity cannot happen fully until after that point.
There are documented health benefits to women who have had children and there are documented health problems that women who don’t have children don’t face.
Let’s use a metaphor:
When I was 8, I could climb up on the roof of my house and, just for fun, jump off.
My body wasn’t ready for that height of drop and I didn’t have the brainpower to tactically land.
I never hurt myself doing it. It could be argued that the ideal age for jumping off the roof is 8. Some would say, If I could climb up to the roof, I was old enough to jump off. My brother of similar age did the same thing and broke his leg. But it healed quickly, in about 2 weeks, and some say our bones are stronger because of the stress we put on them during the growing years.
At 18, I was 6’ tall and jumping off the roof wasn’t a big deal. I did it all the time to sneak out of the house, there was much less risk, but if I broke something I’d be out of action for 3-4 weeks and it might cause a lifelong disability.
It might seem 8 was the ideal age in some respects, but the odds were much higher that I’d have broken something that wouldn’t heal like my back or neck at 8 than at 18 or even 30.
At 40, I wouldn’t even try it.