At what age are cognitive skills revealed/level out?

My son has cp and hydrocephalus from a grad iii ivh. He is almost 3 and still cannot talk. He does show signs of intelligence like helping to put his arms through his shirt sleeves, taking his bib off himself when asked, clapping hands, etc. He is still making progress, but I wanted to know how many more years I have before he reaches his cognitive plateau.

What kind of professional tests cognitive abilities?