Well, my boys 5, 4, and 2, have NEVER gotten over their obsession with the baby, and he is 6 months! The 2 year old is the only one I really have to keep my eye on, because he might accidentally hurt him--he means well, but will try to pick up the baby--yikes.
I think it might be an ongoing thing and something that will make your life just a liiitttlle bit harder. I just tell myself, at least they like him, want to show him affection, but it does drive me bonkers. I've pretty much dealt with this with every baby I have brought home. I just keep him safe,-- with me, doors latched, baby gates, whatever works!