The term "daddy" was originally meant to be a gay man who was also a senior citizen, that is a man who is either retired or on social security, early to mid-sixties or older. It was separate from the term "sugar daddy" -- a man with bucks who supports a young man -- although the two can sometimes be the same.
Now the term "daddy" has pretty much become meaningless and simply means an older man. This doesn't necessarily mean middle-aged, either. On the gay dating site Daddyhunt the "daddies" often look like 32-year-old porn stars (and often are). But guys in their twenties think of these thirty-somethings as "older men" -- in other words, daddies. Of course this is ridiculous. The site Silver Daddies and some well-known "bear" sites actually feature attractive mature men -- mature meaning at least forty and frequently older, including seniors -- but Daddyhunt is pretty much just a silly marketing tool. It's from the same people as Manhunt and isn't much different.
I imagine in time the whole "daddy" concept will go back to what it meant originally. In any case, it's just another indication of age discrimination -- the truly older guy can't get a break.
32-year-old "daddies" indeed!