Well dd "got it" sometime between the age of 2 and 3.
I have no choice but to practice it with her 18 month old brother, since they fight over the same toy at times so I have to say "Ds it's your sister's turn now, give it to her", taking the toy off him if he doesn't volunteer it himself. He screams blue murder sometimes and looks really hurt and put out. I then say "Good boy for sharing, it will be your turn soon", and insist that dd returns it to him for "a turn" after a few minutes.
I don't feel I have a choice but to to enforce sharing and turn taking, even if he doesn't quite get it - because I have to demonstrate fairness to his slightly older sister (who is not old enough to accept - "but he's a baby, let him have it if he wants it" arguments)