I saw MANY children of various ages with "leashes" at
Disneyland. I think most people see it and understand fully why parents are making that choice. It is just too easy to lose a child in a heartbeat there. It's simply too crowded. I wouldn't give a second thought to putting a leash on a six-year old, if there was any question in my mind about whether he would follow my instructions, stay with me, and know how to get help if something unexpected happened. If the child is not mature enough to do these things, it doesn't matter what his age is.
I thought I'd just add one other thing that I may have posted elsewhere. I made a laminated luggage tag for our son. He's seven, but he has autism, and might not "perform" under pressure. In fact, I wasn't sure I could rely on him to remember our room number or any of the details of our trip. (I bought a leash for him, but didn't use it. We carried him a lot, and he will still accept a stroller, because he's very small.) So, I typed up something that had his name on it, our names, our home phone number, our cell phone numbers, my husband's pager number, and the number of the family radio channel that we would be monitoring. I also put the flight dates/numbers/airline, and the dates/hotel/floor where we were staying. I also printed the instructions for using our family radio, and had him carry the radio in a pocket (some of the time, anyway!) Every time we left our hotel room, he wore this tag attached to one of his belt loops. It was quite large and dangled out so someone could see it if need be, but you couldn't read any of it, without getting very close to it. It could be modified for a child to wear on a lanyard, but I was afraid our son would fool with it and remove it, so I just kept it attached to one of his belt loops.
It gave me some peace of mind that, in the event he got lost after everything we did to prevent it, someone would help him to locate us pretty quickly. Maybe something like that would put your mind at ease, too?