As guestDTM said right at the top, learn them while you're young.

Then don't ever change your style!

I have all these English trad songs that are easily called to mind, but I've become lily-livered in my old age, and the sight of the audience slitting their wrists.....

I CAN learn new songs, with serious effort, but they only stick for a day or so. Sometimes after a week I can't even remember WHICH song I last learned.....
Seriously, there seems to be a finite size for my lyrics memory, and I've reached it. It used to be so easy.....