Yeah, people thought my husband was "really young", to become a dad in his early 30s. But he's out there playing ⚽ with his son and nephews, and he's like, Yo... 50 is ancient. 😅 

You have to think about how fit you are likely to be... later on. If you're a parent by 25, then you might be a grandparent by 50 and a great-grandparent by 75. If you start at 35, you have to add a decade to each following generation. More, really, as children tend to adopt the reproductive habits of their parents, so you'd be a parent at 35, grandparent at 70, then probably dead.