CORRECTED "Peak Age" for swimming fast

Masters' swimmers have been proving it for years and Dara proved it with an exclamation point in Beijing that you can go faster at 40-something than you did at 20-something. As someone 15 days older than Dara, what I wonder is this: At what age will the aging process take over and, despite whatever training you do (& without resorting to drug enhancements), you will slow down?