I think there's an awful lot of generalizing going on in this thread
Not all young mothers obsess about material things, and not all older mothers are mature or self sacrificing. And staying at home is not always the self sacrificing choice (though it sure feels like it sometimes!), or the better choice. For some women there is no choice.
Generally though there does seem to be a move towards people in their 20s viewing themselves, and being viewed, as not quite grownups yet. Like how the media will sometimes refer to Britney Spears as a "young mother" as though her age were enough to explain away any parenting snafu. Plenty of us who began having children in our 20s (or younger!) read our car seat manuals
You know how people say the 50s are the new 40s and the 40s are the new 30s and so on? I guess that pushes the 20s down into the teenage years
I think that more and more, people delay having children because they don't feel ready for (or want) the responsibility rather than because of financial concerns. Americans are more financially secure than previous generations, but are having children later. And less of them.
The US is actually very close to not being able to replace ourselves. I'm trying to do my part...up to 4 so far