1. medieval era =/= now
2. life expectancy is much larger now then it was back then, small pox were also a problem. The only reason for us to be living is to reproduce and survive, that's how nature works. The life expectancy was low then, so younger people needed to reproduce. That is most likely why 11-13 yr. olds had babies then
Also bear in mind that back in those times it was 11-16 year old having children with men in their 20-30s.
And it was a cultural thing for girls to have children young.
Girls were basically worthless, being less efficient as builders or farmers and therefore fathers would want to sell them off.
Girls are only useful when they're having children, so you want to sell them when they can do that.
Girls start puberty several years before boys so you sell them then, once they've had their first period. That means around the ages of 10-14.
It was as much a social thing as it was a necessary thing.
And society has changed now and it's no longer acceptable for children to have babies so young, nor to have love, with someone older OR the same age as them. It's not something to promote or even condone.