I have a male friend who wants to know if he is too old to start ballet. He is over 50, but very fit. He's being coming to my Pilates class and seems to be pretty flexible too.
I think he feels self-conscious as an older man.
Does any body have some advice to give? or can share some experience?
He also wonders what he should wear.

I think that it is wonderful that your friend wants to start ballet classes. This will make him more even more fit. I would say that he would be fine in a pair of sweat pants and a tee shirt to start out. He will need to get the proper shoes and a dance belt as well.

Good for him!

Mick Jagger started Ballet a couple of years ago and hes in his early 60's now!

I agree, it's never too late. Aside from the health benefits, it's also a chance for mental growth.
Depending on the size of the company he chooses, he may also be pretty much guaranteed a part in certain productions, such as The Nutcracker (Herr Drosselmeyer {sp?}) and Coppelia (Doctor Coppelius {once again, sp?}). He might not want to be involved in productions of course, as they are almost as much of a commitment as the ballet itself, at least for a certain period of time.
As for what to wear, different companies have different dress codes, and sometimes even different teachers within different companies have different dress codes. He should be able to get documentation that will include the dress code, and if he's uncertain or uncomfortable with any part of it, he will most likely be able to discuss it with his possible future teacher. Good luck to him!