I had a first cousin once removed who was a twin--her twin sister died at birth but she lived to 96. Their mother was 30 when they were born.
My grandmother and her brother were twins--their mother was 26 when they were born. I have a second cousin who had twins when she was 26. One of them is the father of twins who were born when their mother was 30. My other grandmother had younger sisters who were twins, born when their mother was 34.
The reason? First twins often deliver early.
Second, something called twin to twin transfusion where one twin gets more nutrition than the other.
Third: the twins get tangled or have an abnormal lie (sideways).
Four: after the first twin delivers, the second might go sideways and not deliver. Or the placenta might detach from the uterus getting smaller. Or the umbilical cords might tangle up.
Esau and Jacob's delivery in the Bible tells the story of on such delivery. Luckily they must have had a skilled midwife, because it can result in a dead mom and dead babies.
Usually if there is a problem they do a Cesarean section, but in the olden days they would go in and turn the baby to deliver it.
A story from Africa: One of our sisters was called to help with a home delivery that wasn't going well. She delivered baby number one but then a hand came out, and she wasn't sure what to do (she wasn't a nurse but a teacher). So she put the hand back inside, and told the family it was time for her to pray; she went home and checked the books on how to turn the kid around for delivery, and then rushed back and delivered the second baby. It was a boy, who later became a priest and always called her Grandmother....