At what age do cockatiels stop growing? Rocko is nearly two years old and he got a lot bigger.

Babies are almost fully grown when they leave the nest at the age of 1 month. They continue to grow a little and broaden out some for a few months, but Rocko should be fully grown already. Maybe he's standing taller or holding his feathers in a way that makes him look bigger.

I dont think so his hoodie is now even a little tighter and he always stands tall and proud lol.

I do notice difference in size between my grownup parents, their older children (1 yo) and their younger children (9 month old). The younger babies are still a bit smaller than their older siblings, who are a bit smaller than the parents.
As for your question - I don't know when they stop growing.

Make sure he's not getting fat lol. That's not any healthier for birds than it is for humans.

Does his hoodie ever get washed? If so it's possible that it shrank a little.

His hoodie never got washed and no hes not getting fat lol hes very fit.

Maybe he's getting more muscular lol. If he's been doing more flying lately that would do it. Or maybe he really is just finishing his growth. It seems pretty late for that, but every bird is an individual and they do things at their own pace.

Also Tiko stepped up for me well she always does when on the floor why is that? Anyway ill make a seperate thread.