I recently was browsing a forum where someone was wondering if it was worth starting a career in mathematics in their early 30s and linked was a page where the National Science Foundation listed some interesting statistics as to time to doctoral (and some masters) degrees varying by field. Its worth a glance if you want to know how mathematicians compare to other fields as far as median age that people receive their doctoral degrees in other fields, as well as trends throughout the decades as to when people matriculate. Another interesting statistic is the number of degrees conferred in a specific year.
One thing I found interesting is that the median age for mathematicians and physicists to receive their degree is 30.3 (data from 2003) which is the second youngest — the median age for chemists is 29.6.
More info can be found here: http://www.nsf.gov/statistics/infbrief/nsf06312/