An article in today's WSJ* cites a study that shows a declining belief in Santa with age. The results are:
Age % believeing in Santa
*Behind a paywall. Trust me, it's there.
**All respondents over 20 years old affirming a belief in Santa also confirmed that they were either employed or supported by a government agency. (O.K. I made those last 2 lines up, but it makes sense doesn't it?)