Maybe I'm not yet old enough to know, or just not holy enough, but... I'm not sure that sin, on the whole, gets easier to resist as we age. Yes, a few lusts might become less enticing. But others, if they've been indulged for a lifetime, just get more firmly entrenched as we become set in our ways. The time to repent is now; the earlier in life, the better. Fight sin before it grows deeper roots.
(And whatever Piper may have said, I'm sure we would agree with the need to fight sin now.)