Unfortunately, playing the piano is not like riding a bicycle. We do, indeed, forget. The fruits of all those years of practice, music festivals, and Royal Conservatory exams have not stuck with me.
A few years ago, I got a very nice keyboard, and thought that I would sit and play for the sheer enjoyment of it as I used to - perhaps I would have to back up a grade or two, maybe start with seven or eight, but as I backed up farther and farther through my old music books, I realized just how rusty I was. I couldn't believe how much I had forgotten, how much work it would be to bring back the skills and knowledge. Serious practice.
I have decided to play it again and have ordered some books, dug out some sheet music and practice books and am going to sit down to play scales and chords and figure out those keys again - how many flats in that piece?!?!