As is the case with most high-end electronics, the older they are the better they were made and the longer they will last. As others have stated it also depends on how the radios were treated,stored,maintained,etc... I have seen new radios owned by some folks and after a year or so they look like they were put through the mill !
I have some older Futaba 8U's that still work fine and my '76 Kraft I converted to 2.4 and it's working fine as well. I also have a Spektrum 9 and I hope it lasts as long too .....so far so good.