I don't think the biological clock discussion needs to focus just on mothers. There are examples of both parents not taking responsible precautions against pregnancy when they are not currently in the financial circumstances to support the nascent child.

I think S_C has a really good point, I wonder why I never considered this before.

My older sister was raised by our mother, she became pregnant at 20, she's now 29 and has two 9-year-old boys.

My little sister and I were raised by our father, I'm 27, she's 26, neither of us have ever come close to having children.  Sometimes we wonder if we even have biological clocks.

I honestly figured my older sister was just more maternal, she saw our mother raising our half-brother and was told that it is good for women to grow up and have babies.  Our dad told us that we can be anything we want to be, he mentioned a lot of things, doctors, lawyers, teachers, anything and everything, except wives and mothers.