We went to Disney Paris when they were 3 and 5 which was magical for them. I couldn't face taking a child under 8/9 long haul, flying economy! We took them to Florida when they were 10 and 12, which meant they could go on most of the rides and were old enough to remember the holiday for the rest of their lives, but weren't grumpy teenagers!
We took ours when they were 8yrs and 16months. Obviously the one who was 16months doesn’t remember the holiday but the 8 year old had a great time and was able to go on all the rides bar 1. We’re going again this year, they’ll be 14 and 21, so the little one will remember it this time.
7-8 I reckon. Oddly there's not much in terms of play areas for under-3s (universal was better than Disney on that score), at 7-8 they can probably go on the majority of rides, can carry their own lunches in their backpacks and stay up late enough to see the parades in the evening.
We went when DS had just turned 7 and it was a great age. He was old enough to cope with all the walking, but young enough to still get the magic at Disney.
Oddly Universal also worked really well as no one really goes to Universal for the younger age group, so apart from a queue for the Harry potter stuff we waltzed on to everything with no waiting.
The only trip that didn't work was Kennedy Space Centre as DH wanted to spend a long time going round everything and booked us onto the extra trip to the mission control bit ( which looked just like my Dad's office in the 70s, except they did cool things there). DS was just far too young to get it and a lot of the exhibits weren't particularly child friendly. I think he'd love it now age 11-12.
DS loved the plane trip as on shorter journeys as we always go with low cost airlines so there's no entertainment. He was delighted with his console on the chair and I spent many a happy hour playing tic tac toe with him on it.
Took ds to Disney Florida aged 3. He loved it, still talks about it almost daily a year on, and there was absolutely plenty of rides and activities for him.
However, he struggled with the heat and the queues. And when he found something he liked (eg a fountain jumping out of the ground) he just wanted to stay there all day, which was sweet, but frustrating when he can’t understand that there’s something even better around the corner- it felt like I was dragging him on all the time, but it was needed in order to show him all the great stuff. An older child with a bit more comprehension of what they can’t yet see would be better!
Apart from a young baby, there is no best age as children will get something different out of it depending on age group - younger ones interested in characters and rides, older ones the rides, and the shows for all ages. (We've been several times over the last 7 years)