my kids are 8 yrs apart so we've had to work aroudn the age difference every trip to WDW, USO and any other parks we've visited.
Since around DS turned 12 we let him go off on his own to do the rides DD can not.
He always have a radio with him, but we make a backup meeting plan just incase the radio isn't working for some reason (had that happen a few time, from batteries dying, to not being able to get a signal, or forgetting to lock the channel and it accidentally getting changed). When he was 12-13 we would only let him go off for a short amount of time, about an hour for each park. Now we spend an hr or so together in the AM, separate and the meet up for lunch. Spend another hr or so together and separate until dinner (or park closing time if it is early).