Again talking about Emirates, most cabin crew there are under 40.
They start off working Economy then after around 2.5yrs move up to Business then after another 3 years tend to move up to First.
From my chats to cabin crew during my travels, most people leave after about 6 years and go back to their own country.
The oldest cabin crew I ever encountered was flying from Orlando to Chicago in First about 6 years ago with American Airlines. The crew there were late 50's and have been in the airline for many years.
It gave a homely feel to the cabin and it was like having my mum looking after me