Like I said, Garp 12 years ago was 66.
But maybe I'm interpreting this wrong.
Kuzan matching Garp's punch happened before that tho, but 66 is close from prime anyway, Kizaru is 58, Kaido is 59, BM is
68
It's only when you reach your 70s that there seems to be a decline in this verse.
Anyway as I said, sound was used to show Kuzan's strenght growth and progressively matching Garp's punch, Sound of Garp's punch stayed the same in the three panel. I don't think Kuzan only matching Garp's punch because it got weaker instead of him reaching Garp's strenght when he started training is what that scene was aiming for.
Anyway this is without haki, this does not mean Kuzan would hit as hard as Prime Garp in a fight lol.