I think it depends on how many times you can afford. Mine have gone multiple times as youngsters. My oldest vaguely remembers being there at almost 4, but my middle child went at 5 and doesn't remember much now that she's 8. Still, there's nothing like Disney Magic when you're a preschooler or younger. I think the experiences affect them even if they don't remember it. Our oldest went at 8 months for the first time, then we went with them at almost 4 and almost 1, then again at 6, 4, and 6 months, then again at barely 8, 5, and 3. Only my oldest really remembers much from our last trip three years ago. We had a great time every time. I will say I'd rather go when they're in diapers or completely diaper-free. It's the worst when they're potty training, and by all means if you go when they're potty training put them in a pull up all week and don't ever depend on them to make it to the toilet.