The Wave by Algernon Blackwood

Between twenty and thirty a man rises through years reckless of power and spendthrift of easy promises. The wave of life is rising, and every force tends upwards in a steady rush. At thirty comes a pause upon the level, but with thirty-five there are signs of the droop downhill. Age is first realised when, instead of looking forward only, he surprises thought in the act of looking -- behind.

Of the physical, at any rate, this is true; for the mental and emotional wave is still ripening towards its higher curve, while the spiritual crest hangs hiding in the sky far overhead, beckoning beyond towards unvistaed reaches.