He is very well kept, has no vices and is by all reports, a gym rat. He has taken little damage, and has had no ring wars. His style doesn't depend on speed or reflexes but rather on excellent fundamentals. If people are waiting for him to get old overnight, they will be waiting a while. Even at 38 I see him as being a handful for even the best.
He will aged well i think, has a good style for later years however he will lose some of that killer instict and danger he has, especially if he moves up.
Fighters who rely on reflexes and speed are the ones who get "shot" quickly. Golovkin relies on setting traps, countering and opening his opponents up. His power is also the last thing that will go too, so he'll always have that to bail him out of trouble. What might work against him is if he unifies, moves up a division and suddenly has to fight 6ft+ opponents who aren't easy to bomb out. His training camps will have to be longer, his body will take longer to heal and he'll take more punishment in the ring also. That is the only way I can see a prematurely ageing career.
Don't take much punishment in fights or waste much energy could go for years plus his power means short fights.
With that kind of opposition he could fight to 60 years old. He's not really a speed demon and relies on very solid fundamentals so that type of style is good for longevity, plus as others have said, he hasn't taken any punishment into his mid 30's.
He will be shot immediately after Saul Alvarez beats him by KO in 2017. The haters will not give Canelo his dues, just like they haven't until now despite having the best resume between 147-168 out of all the top fighters.
Again, my thoughts exactly. They won't give Canelo any credit and will likely cry how he waited for Golovkin to get old.