I have flown with Iris. I will tell you that she does not look her age and seems to do as much and as well as the younger (well, younger than her) flight attendants on the flight.
She is based in SFO and seems to do the NRT flights the most.
Pilots, the FAA says, must retire at 60 due to declining mental acuity--not because of physical ability. There are no physical requirements to the job, other than they must pass a Class I physical every six months. You've seen a lot of the pilots. That physical is not that hard to pass (except the EKG, perhaps). That's to hopefully make sure that they don't drop dead while flying the plane (which has happened).
The mentail acuity mentality would be a hard sell in relation to flight attendant duties. Physical fitness tests have never been a part of flight attendant recurrent testing. There are certain physical factors associated with the annual recurrent training (pulling the window exit out) so they have to at least be able to do that.
But I see no reason to have a pre-set age at which flight attendants must retire. In fact, I will not be surprised to see the pilot age raised to 65 in the not too distant future.