I don't think memory and size of repertoire are related. You're either blessed (or cursed) with a good memory or you're not and whilst it undoubtably declines as you grow older it doesn't seem to have happened to me..........yet. (I'm 68 by the way) I draw upon a 'working' repertoire of about 150 songs but felt quite humbled by Bonny Sartin of the Yetties mentioning, at their Sherborne club, that he knew 800 songs. Everything they performed that evening (and some were requests) he sang from memory.