What is the Max age for a RAF employee to rejoin the UK RAF?

Depends on the trade in question, most have an upper age limit of 29-35 years for new recruits, but medical officers for example, are accepted up to 55. I believe they have a degree of discretion on this if you have previously served but no hard and fast rule, would depend on the needs of the service, your prior record and current fitness. Bear in mind you would probably be looking at a minimum 9 year enlistment with retirement at 55 max. So most probably anything over early 40's is out of the question for the regular RAF. RAF Reserves recruit up to age 50 (56 if you have previous military service) might be worth looking into.