Congrats on the new baby!
We took DS for first time to WDW at 10 months old. I thought he would be way too young to enjoy anything, but honestly, he was great the entire trip, never cried at the characters and had a great time. He is now 20 months old and we are taking him to DLR for his 2nd birthday.
I don't think that there is really a perfect age. I think that if you take an infant you should plan on doing things at a little slower pace to accommodate them, but you can still have an amazing trip and create wonderful family memories. I love all of the pictures I have of DS seeing Mickey for the first time. I'll cherish those pictures and our first vacation with him forever.