Serco UAE is not bound by UK or US law. It is quite legal in the UAE to discriminate based on age (or sex, religion, nationality, and many other attributes). If you have any doubt about that, just check out a copy of the job ads in any UAE based newspaper or website. Having said that, Serco (UAE) has occasionally hired ATCOs who are past the company "cut off" age but only if those people are already personally and professionally known to Serco. Otherwise you will NOT get an interview.
To be clear, I am making no comment whether Serco's policy is wise or even appropriate, but it's their show.