Actually, science has proven we have "internal clocks" that speed up as we age. In a non-controlled, unscientific experiment, people were stopped on the street and asked to count to 1 minute by seconds. Younger people almost without fail, got to 60 before the stopwatch did, and therefore perceived that time was moving "slowly", while older people invariable got to 60 after the stopwatch, perceiving time as moving "faster". Science hasn't figured out why, but time really does "speed up" or at least we perceive it as speeding up, as we grow old.
And I'm now 52 but still feel 18!