Is my little one advanced, or is my older one slow?

As the mother of 4, I can tell you one thing for certain, children develop at different rates... However, if this bothers you enough to ask, don't be afraid to ask your ped. Be prepared to be blown off. Have a list of behaviors and ask for an idea of when they should be met. The biggest problem I had in this area turned out to be caused by the older wanting to be treated as the baby. Try reinforcing better pronunciation with a simple chart with problem words, and a good supply of gold stars!