Nobody seems to have addressed the primary question here which was;
"HOW OLD IS TOO OLD?"
WHat you need is some sort of reliable indicator to let you know when you are too old to windsurf otherwise you could be sailing along one day and find you had died yesterday, and that's got to be embarrassing. Nothing worse than a corpse on the course to dampen everyones enthusiasm for the sport.
Anyway, after much study on the subject over many years I have arrived at the following conclusion.
I have noticed that as you get older and get less able to do things, the mind compensates by growing a bit dim and giving you the altered perception that you are actually getting better at things and that your body shape is improving. This is amply demonstrated by the fat old asses I see strutting down the beach in their budgie smugglers (aka CJs (cock jocks)) with their gut hanging over the CJs which they used to wear when they were 14 and kept in a cupboard for the "inevitable" day when they "got back in shape again". Only they never did get back in shape, their mind just went a bit saline and thought they did.

So if this sounds a little bit familiar to you then I think you can take it that YOU ARE NOW TOO OLD FOR WINDSURFING!!!!
For myself, my CJs are still safely in the bottom drawer so I'm obviously not too old for windsurfing. ( although I have tried them on a few times recently. Hmmmmm... Might be a bad sign.)