With greater prosperity people do not feel the need propagate the world with progeny because they will be able to save enough for themselves to pay people to take care of them in times of infirmary (or depend on the government).
Thus, in the US, Old Europe, Australia and Japan the birthrate has decreased to the point of receding population unless immigrants from poor countries are allowed in.
I do not believe this to be a difference between Baby Boomers and Gen X/Y. It is the perils of prosperity.
Being 40 with kids under 10, I would encourage couples to have kids in their 20s. They can wear me out!
Being 45 (my husband is 50) with a 7 year old - I can totally realte. However, I'm not sorry I/we waited.....We have much more patience, and definitely more time to devote to her needs. By being a bit older, it would be nice if I were working right now, but it is not necessary and so I am home when she gets home from school each day and am not rushed with trying to deal with such things as dinner, homework, baths, etc.
If I truly felt prosperous and my assets were secure from government seizure (mostly due to demands of the rapacious retiring Boomers), I would have LOTS of kids.
As it currently stands, I will make 6 figures this year and still don't feel I could provide adequately for children.