Something to keep in mind in this discussion, especially lately, is that the child needs to be mature enough to be waiting alone for potentially a LONG time if a ride breaks down while the parents are on it. This is happening so often now that you have to assume any ride you get on might stop mid ride and you may be stuck for a bit before a CM gets you out. This has happened to us recently on a few rides. This is where it is immensely helpful for the child to have a cell phone so you can communicate with them.
If your kid can't be okay alone for at least an hour, don't leave them waiting for you outside a ride. Wait until they are old enough or skip that ride altogether.