Yerushalmi (Chagigah 2:1): Teshuvah is accepted even until Dichucha Shel Nefesh (he is crushed and depressed). Sha'arei Teshuvah (1:9) - however, the best Teshuvah is in his youth, when he has his strength, and he overcomes his Yetzer.

Rashi: You bring afflictions on a person until he is weak and close to death.

Radak: It is until he is crushed in his old age. I.e. the strength of his youth will not help him, for You will return him to be crushed.

Pesachim 54a: This teaches that Teshuvah was created before the world. It says "b'Terem Harim Yuladu