I was 7. The perfect age, I think (although it depends on the kid!). I was old enough to get on all the rides without being scared of anything (like the snow white ride), tall enough for the height restrictions (loved BTMRR!), but young enought to BELIEVE in the Magic (still do, really!). And I'm sure my parents liked not having diaper changing stops or needing to push a stroller around.
But my BF is going for the first time this December at age 32. I swear he's more excited about going than I am. He grew up poor, and never had a vacation of any type, let alone WDW. So he say's he's having the childhood he never had! I'm working very hard to make this the perfect trip for him.