Paradoxical as it may seem, to believe in youth is to look backward; to look forward we must believe in age.- Dorothy L Sayers, born on this date in 1893
Isn't 'youth' an 'age'? I don't understand this statement. Perhaps it lacks context.
The contrast is between youth -- "the time of life when one is young; especially: the period between childhood and maturity" -- and age -- "an advanced stage of life" (both definitions from Merriam Webster).