I have been doing the Sunday Times Crossword since my early twenties since I enjoy the challenge (but sadly never won). The prize is a Cross Pen which I have probably paid for many times over in postage stamps. Indeed, I started giving blood as a student at 18, and have just passed the 50 pints mark which entitled me to very same Cross pen with a nice enscription. Nuff said? Indeed on passing that landmark I sent in a question to ST along the vary same lines you did (no reply).
However, I have read that doing crosswords can ward off Alzheimers (as can playing chess apparently). In addition, I now get paid for compiling cryptic crosswords. Not great money but 100 pound a pop for 3-4 hours work. Sadly so far only one a month so cannot give up the day job but a great way to fill dead time! As is doing the blasted ST crossword!