I'm 59, but don't have that memory problem: I just learned over 100 pages in that Bellowhead piece. It can be done. My problem was that a careless Jordanian academic gave me her laryngitis a week before the gigs: technique carried me through.
I think it's a matter of pride: if you're not ready, don't do it. Know why and anticipate the problems next time. Performance nerves are just a cop-out: you should be attuned to the venue and audience, but those are empathic nerves, and should help your assurance: anything else should be ignored. Your first task is to grab the audience' attention: there's nothing worse for you than a gang more concerned with fighting their way to the bar than what you're doing. In such circumstances, there's an argument for busking till ready, doing something you fancy rather than something in the program. If the organisers are so disorganised as not to have got the audience settled by the time they want you "on", then that's their problem. But even then, you still have to grab them, and that needs musicianship, and the root of that is knowing what you're doing. In the worst of cases, recognising a failure can even be a positive experience.