Golovkin is 35 and he slowed down as he showed in Brook and Jacobs fights. This is the main reason the fight is made. It's more of a business.
Odds are still in GGG's favor.
It would favor Canelo more if he fought for the IBF since he's younger and can lose/gain weight better and faster. But in the end they are both going to weigh around 172ish
GGG has the IBF belt and makes the 10lbs limit easy.