It's the wall because we're here to procreate and build families. If you don't want kids, then you're out of the discussion. The wall is about viability for having kids and building families.

A 35-year-old having his first kids is looking at 30-35 years before the grandkids start popping up. He or she will be 65-70 then. That's acceptable.

A 45-year-old having his first kids is looking at being a 75-80 yr old granddad. Or dead. Not acceptable.

A 55-year-old having his first kids is looking at being a 85-90 yr old grandad. Or dead. Not acceptable.

No matter how we try to fool ourselves, 35 is the wall for us all.

I've had discussion with my girlfriend about how things could play out for us. She's early 20s, I'm getting towards late 20s. I'd like to start a family at around age 30. Problem is, she says she won't be ready until she's at least 30, which makes me a few years older. I wouldn't want to leave it any later than that. I want to be able to run about with my grandkids. Not grouch at them from my nursing home bed.