As a dad to 2 kids (became a father at 28) I would strongly urge people to wait until they were 30-38 before having kids.
Given the great innovations in this space I'd even say 40 is a great age to have kids - though it gets harder with age beyond this imho.
And grandparents are a major factor, too. They are a great help, and also they really enjoy being with their children. Supposing my kids also wait until they are 38 to have kids, my odds at enjoying quality time with grandchildren are greatly reduced.
I'll probably be almost of pension age when my kids finish their education. That's another thing where I worry I might not be able to help them out as much as I could if I would still be earning money and connected in the business world.
It would perhaps be different if I had toiled and saved money all the time up to age 38, but I didn't. If you have kids, odds are you get more serious about those things.
I do take issue with the statement that young people (people in their 20s) have energy "in abundance."