It's fairly common, in painting as well as photography, to put a limit on the age of the piece. The art club I belong to does it, one of the stated reasons being to encourage artists to do new work. We also don't accept work entered into previous years of the same show. The hitch with all this is that it's pretty much honor system and/or relying on the fuzzy memories of the committee. Didn't we see those orange eggs in the purple frying pan last year?