You presumably know far more professional developers at a later stage of your career than someone fresh out of university, so you have a far bigger pool to find someone out of.
Most of the people who are just finishing university and getting into YC aren't people who would be struggling to find jobs, these are people who are choosing to work for a startup over going to work for Google or Goldman Sachs for a six-figure salary.
The fact that someone older is more likely to have a mortgage and family commitments and thus be unable to relocate for three months and give up their salary is far more likely to weed out older candidates than the co-founder requirement.
Most of the people who are just finishing university and getting into YC aren't people who would be struggling to find jobs, these are people who are choosing to work for a startup over going to work for Google or Goldman Sachs for a six-figure salary.
The fact that someone older is more likely to have a mortgage and family commitments and thus be unable to relocate for three months and give up their salary is far more likely to weed out older candidates than the co-founder requirement.