It is not about EU and asian airlines hire younger people, UA and other US carriers hire younger people too. It is about how long the airline had been existed, and also the retirement age.

How would you like saying you had a job you like, but they say you have to retire by 40, because you are old , then you left jobless in the middle of your life, no one would hire you ?

Many Asian airlines particular has a very young history, like BR and OZ, they establish in the late 80s, KA in early 80s, compare to NW and UA in the 30s. Many airlines that had a union has some older FA as well, I have seen many ISM on CX are in their late 40s, some JL and SQ ISM are in their 50s as well.