So you're taking an average middle aged man, worrying about conceiving a child with a younger wife at the end of HER fertility, and then fast-forwarding how old he'll be at different stages of the child's life?
I guess the only conclusion is "sometimes you have old parents". This is true for many "Gen Z approved" equal-age marriages as well.
What if you conceive at 42 or something? You'll be 60 when your child is graduating High School. Such is life.
But more realistically -- your wife will be AT LEAST a couple years younger than you. What if you have a child at the very end of her fertility? You'd add a few more years then. The dad is going to be older.
And how healthy/active is a man at 60? It HIGHLY VARIES. Some men are in "scooter ville", with a handicap sticker on their car, no longer able to walk around the big box store on their own power. Others are still playing sports. It all depends on genetics, lifestyle, career, and how they took care of themselves.
So no, it's not required or even "average" to be the energetic, youthful 20-year old dad who is as active/healthy as a child himself.
Yes, if you happen to be the youngest child, and your parents conceived you in the late autumn of their fertility -- then yes, you are going to have older parents.
Some people have to bury their first parent before they turn 20; others have their parents with them for almost their whole life. In fact, some parents have to bury their children! It's just the inequality of life.
Even if you have parents 45+ years older than you, some people have *additional* crosses to bear. For example, I know one woman who is in her late 40's but she doesn't look a day under 70. No exaggeration, either. Did she use meth? I know she's a smoker. She had all her teeth removed and looks like she doesn't bother with false teeth. She had a daughter when she was in her low 40's. Imagine that daughter's situation.