First off, I don't think any age is completely out of the question, but here's my general feeling on age at WDW. Take it for what it's worth, which is probably not much.
- In-utero: Okay, but follow those preggo advisories!
- Newborn to 1yo: Probably too young, too much stimulation.
- Terrible Twos: Possible, but at a slooooow pace, lots of breaks, and a lot of stroller/diaper bag hassle. :babyroll:
- Three: Similar to above, but potty trained?
- Four or Five: Close, but a lot depends on your child's maturity. Limited ride options, and you're probably still wheeling that stroller.
- Six to Ten: The perfect age in my opinion, though the younger ones will need a slower pace and may be too short for some rides.
- Ten and up: Disney says your kid is an adult, so now you're all set!