I think from watching my nephew (now 6) & my niece (now 22 months) & how my sisters/family have raised them, its better from an early age.
Iv realised, even with my baby (8 months) that even though he may not know the meaning of hayi (no) he understands that Im not happy from my tone. In the beginning he wld look at me, do it again, I say hayi again, then he would come to me whimpering for a hug!! These days he ignores me though, he will literally look at me square in the eyes & carry on with whatever Im saying no to. When I go to remove him, he cries even before I reach him coz he knows Im gonna take him away & put him some place else.
Then he stubbornly crawls back to where I removed him from or he throws a tantrum, crying & throwing his body backwards.
So he obviously understands & he's starting to test boundaries.
Im pretty consistant though & the nanny also is, so now he automatically dsnt go to the restricted areas when he can see we are looking. He does go though if he thinks we not looking.
He also scratches, pulls my hair, hits my chest & bites my face. He's not being aggressive though, he's being playful.
What Iv learned with him though is if he pulls my hair & slobbers all over my face, bites me, its either he's tired or hungry. If I dnt do anything bout it, he'll starts crying.
Sometimes he just scratches me, but there really is no aggression involved