We only have one son, and my husband was 42 and I was 41 when he was adopted. He is 9 now. Except for the occasional rude person who comments out loud whether we might his grandparents, everything else is OK. In terms of energy level, we run circles around people who are 15 years younger than we. Actually, while we were at WDW, we ran circles around the teen-aged sons of some friends who accompanied us.
I think that you will find, as others have pointed out, that many people in competitive fields defer child-bearing until later. I know that when I was 27 years old, and working as a chemist in a research lab, that the female director of my division had her first child when she was 42. Even among my colleagues, having a child in one's mid-30's is actually young. I am a physician, and I make the observation that most of the women who were my classmates who had children already before starting medical school eventually dropped out. I can't think of any, off hand, who had children while I was in medical school, but I am sure it happened. By the time a person completes residency and starts in practice, they will be about 29 or so. So, most women in my specialty who have babies are at least 32-35 when they have their first one. So, I am sure that many are not finished completing their family until they are at least the age I was when I adopted my son. Perhaps, in a sense, he got the best of both worlds. His natural mother was 19, so he had the advantage of a robust biological mother. My husband and I are much better off financially now than we were in our 20's, and I like to think, alot more mature in our thinking and better parents than we would have been then.