I believe I read this one in a new book by Larry King, Tell Me More.
I think it came from a fairly old comedian.

A man walks along, and sees an old man sitting on a park bench.
The old man is perhaps eighty, and is crying his eyes out.

The first man approaches the elderly one and asks Whats wrong?

The elderly fellow replies. I just married a twenty-year old
girl, and…

The first man prods him: And? Do you have troubles with…

Oh, no, no. I have incredible stamina for a man my age, and we
make love twenty-three times a day.

The first man, obviously surprised, asks So whats wrong?

I forgot where I live! bawls the old man.