I recently read an article on memory loss that said people are at their peak in their 20's, and brain cells start to die off after that. It also said that the die-off can be accelerated if you have ever had any kind of traumatic head injury, even if it wasn't severe enough to even see a doctor, and that bumps on the head are cumulative.

As for forgetting the words in public, I have seen many, many performers do this, and trick is to simply carry it off gracefully. Most audiences are very forgiving. I once saw Andy Irvine doing a request he hadn't sung in years. It was a story-ballad, and he sang the wrong verse in the wrong place. He apparently realized it after the first line, because he made up two new lines that rhymed and made sense while singing the second line, and never missed a beat. Anyone who didn't know the song would never have guessed, but of course the audience all knew it better than he did. When he finished, he shrugged and said something like, "Oh, well, messed that up." But it was as fine a save as ever I saw.