[EvanK]

Because they can. We live longer now than we ever have...We don't have to be married and having kids by the time we're twenty, before the plague or dysentery gets us. A female friend of mine just turned 30 and she's just getting into an actual career (and she's perfectly fine with taking her time).

[ergo98]

"We don't have to be married and having kids by the time we're twenty, before the plague or dysentery gets us."

There are many choices for life, and choosing never to get married and never to have kids is exactly the right choice for many people. People don't need to build an army of labor to tend the farm or whatever anymore.

Having said that, the human body is geared to having children young. A lot of women are putting off having children and then finding it either impossible, or learning that the risks of an endless array of defects goes up quite dramatically.

[EvanK]

Even then, we still have options that people haven't always had, like adoption or surrogacy.

[DougWebb]

I think orphans have existed about as long as children have, and adoption has always been there too.

Surrogacy in the sense of "my sperm and your eggs in her body" is new, but men having multiple wives with the younger ones for child bearing is not. So I'd say the concept is old, and only the technology is new.