Has he said so in an essay? On a HN post? To YC founders in person?

See comment above. There was another interview or blog post where he talked about his preference for young people more extensively, but I can't find it. Do you happen to know how many people over 50 have been accepted into Y-Combinator?

I wonder how many people over 50 would apply? IIRC the seed capital they provide is probably small enough that a 50 year old would likely have that in their savings and possibly enough contacts/credit rating to get other funding directly.

The closest I could find was this: http://www.paulgraham.com/mit.html

But all he really says there is that younger founders have an advantage that they can live on less money.

You're thinking of someone else. Paul has corrected people here several times when they say something like, "I'd like to apply to YC but I'm X years old.". Usually his reply is approximately "We've funded several people ten years older than you.". I think the oldest YC funded founder is 50 something. So far.