FWIW, I'm 32. I guess you can go chasing girls in night clubs when you're older, but it's way easier to do that stuff when you're young.
Similarly it's possible for people to wait until their 40s to have kids, but I'd say it's far easier to have them when you're young and have the energy.
I'd be interested in seeing a graph of difficulty of raising kids with age and income -- it probably depends on parenting style, but I think it would (will? maybe.) be easier for me to have kids at 40 with >$bignum than at 18 with $1k, assuming I'm in excellent health for my age (honestly, I'm healthier and more fit at 31 than I was ever before, and hopefully at 35 will be even more so -- not being into exercise early is probably my biggest regret).
This is the correct answer. For example, when your stay at home wife is coming to the end of her rope, and you can decide on the spot to put your kid in a part time day home to give her some her-time, with no need to consider the cost, it makes life a lot easier. Ditto for so many other things in life.
Whether a person should have to do this is another discussion entirely. ;(
Similarly it's possible for people to wait until their 40s to have kids, but I'd say it's far easier to have them when you're young and have the energy.