They can't have children, by the time they could get married she will be no longer able to...

People do have children in there 40's ii's just not advised.

It's extremely rare. She can still have children as long as she has not hit menopause. But I don't think they will be able to have more than 1 child.

Adoption is a thing lol

Menopause more often occurs at age 50+, according to what my mother told me. She had me in her forties, so I wouldn't say it's "extremely rare," but it's definitely more uncommon for sure. The most extreme example I can think of for women having children over age 50+ is an old woman who bore a son at like 85-91 years old in some eastern country. My eldest sister is almost 40 and she recently bore a healthy baby boy a couple weeks early, but she was very careful in making sure that there would be no complications during her pregnancy.

Menopause can hit in 40s too. My mother had menopause at 42. Women in my family (of both maternal and paternal sides) hit menopause before 50. We also start having menstruation between 8-13, so it might be a genetic thing. I have personally never met anyone (above 48) who still has periods though.

Eh, my mom had me when she was forty. It isn't that rare

I don't think she could marry him in the immediate, he's on his 17 years old and by the time he finishes his studies and find a job and think about marriage she wouldn't be able to give him children

I don't understand your analyse at all. They've known each other for years through their previous life and seem already really fond of each other in the current life. There's no reason for them to wait too long. Literally the only reason she's hesitant is because of his age. When he turns 18 it would be legal. She would be 40 and still old enough to have children, there's plenty of proof in the real world with my mom as an example, but the most important part that you've clearly missed is that this is a fictional story and if the author wants her to have a kid, then she's going to have it.

Marriage is a big responsibility and I don't think his family would allow their marriage in such a young age. And btw sometimes I forget that's just a story and the reincarnation itself is a fantasy so yeah everything is possible ╮(￣▽￣)╭

I'd said "more often," which does NOT means always. And I do believe menopause and periods starting/stopping at certain ages varies between genetics of different ethnic groups and hormonal inconsistencies in terms of bodily health. There is a range, but there can't be exact specifics due to whatever conditions involved.