I think that folks that complain about being "too old" for YC usually mean that they have lots of commitments that aren't compatible with the requirements of the program.
But, there are plenty of YC founders of all ages with significant others, kids, mortgages and other financial commitments.
People that are looking for an excuse will typically find one.
It's trotted out to explain why people who are older earn more when they have the same experience. It's why our student allowance in Australia is ridiculously low but our unemployment is much higher. It's used as an excuse for only asking the single/childless employees to work late/weekends/not from home.
You're right, it's definitely just an excuse. People generally chose their life commitments in some way, and using them to justify unfair treatment is just incredibly annoying.
I hope you're right. In fact, I'm counting on it; being a 37 year old founder who's looking to apply (possibly) for the next Winter cycle. Luckily I'm still single and have no mortgage, just a car payment and rent, so I have a fair amount of freedom to make radical changes in my life if I want. :-)