The USA Swimming stats are a little disappointing with the numbers declining steadily for swimmers over 12 years old. There are just slightly more than half 16 year olds as there are 12 year olds.
Here are some 2014 statistics:
www.usms.org/.../reg_stats.pdf
That's a great chart. I would bet 20-30 years ago the highest % of male and female swimmers would have been in the 30-34 age group. It would be interesting to see a similar chart done with only swimmers who compete in meets. All too often swimmers drop out of the meet circuit using the excuse "I can't go as fast as I used to anymore". When I ask them "why do you think there age groups" they usually have no answer.