I've only got one and I've had it's been there since I was about 16-18. No sight of the other 3 though and my mate who's 31 has only got 2, he thought 2 was all he was going to get till I enlightened him that there'd be 4 eventually!
Mine are taking ages too! I have two fully grown, one half-way through, and the fourth one is just coming out now...I'm 29, and they have been growing since I was about 20!
Mine started growing at about 21....they are still growing at 33. I have 2 fully grown ones, 1 half grown and 1 just popping through the gum. Should have them all by the time I'm 40!!!!