Mabel - the NHS one looks to be up to age 50, Anthony Nolan is 40 - we had them round at our work a few years ago and they explained it's because of the costs associated with registration and testing, they need you to then be 'available' as a donor for a good few years after that (not sure at what age the cut off is) and they drew the line at 40.
I'm registered with Anthony Nolan - at the time I had no blood relatives apart from my parents (who would have been too old to donate); I know that I would more than likely be entirely reliant on a stranger should I need a transplant and therefore I figured it was only fair that I was prepared to donate too.