Dd was 16 months. She didn't really seem to get it at first, but then she started wanting to carry the baby. These days if she does something and it makes him laugh (like jumping), she keeps doing it. She gives him kisses and hugs and rubs his feet and says aww. She's not jealous, but she doesn't want him playing with toys that used to be hers. but then even the cat isn't allowed to play with her toys lol.