Our DS's first trip was at 5 months and, to be honest, we wouldn't do it again that young. I'm sure other people will have different experiences from mine, but we found that taking a young baby was more hassle than benefit.
Our DS would not nap in a stroller, so we had to go back to the resort for nap times. He napped from 9-11 and 1-3, and then had to be in bed by 6:00. It made it impossible to really do ANYTHING at the parks, because we had to be heading back to the resort by the time we got into the gates, practically. I know some babies are more adaptable, and it probably would have been easier with a younger baby who could sleep through anything.
Meals were also a mess because he couldn't sit up in a high chair yet, but wanted to sit with everyone else. It meant that someone was always holding a baby on their lap while eating, and DS got frustrated and bored with meals very quickly and would start to fuss. We cancelled most of our ADR's and ate CS as fast as we could most days.
We took DS to DLR at 15 months and had a MUCH better trip! He was much more interactive with the rides and characters, was walking, could sit and eat with us at meal times, and was only napping once a day in the afternoon.
We are expecting our second child in a few weeks and we will not do another Disney trip until she is at least 18 months.
I'm sure other people will have great stories of traveling with an infant, but that wasn't our experience.