I've been performing for a solid 25 years and 50 has stared me in the face for a few years now. I haven't noticed the memory problem yet while singing but I bet I can't sit here and type out every song I know. What ever nmemonic there is that causes me to remember a song from years past I wish I knew. I'd bottle it. I can recall sitting with friends from 20 years gone and suddenly we were singing just the songs we knew at school and hadn't thot of since. The keys were the same and we even made the same mistakes in the same places. I hadn't sung "The Tellin Takes Me Home" since '78 I'd bet but there it was coming out like it was on the set list.
Because I play in an active band I try to bring a new song ready to play about every third practice. And we also have to learn the new Hot Tune to keep the requesters happy. The question is if we choose songs with such care, which ones do we drop when we bring in a new one?