id say normal, my eldest is just starting to grow out of it, tho he still growls and pretends he is going to claw things.
my almost 4yo is still right in the middle of it all, his faves are dog, cat, frog, bubble() and most recently granpa in my pocket (this one gets a bit much when were out and he hops on the pram wheel, refusing to hop off untill i give him a "shrinking hat") next month im sure there will be atleast 3 more added on the list

personaly i think the longer its encouraged, the longer they do it, if i let on the slightest hint of amusement he will continue on for hours, wereas if i ignore him (some days its very hard to do!) he snaps out of it and returns to "trent" when he realises theres no attention.