to be honest, while life expectancy is increasing past a certain point we're not extending 100% completely healthy lives, we're dealing and extending life while not at its peak if you see what i mean?
My grandma is, really can't remember the age, approaching 80 anyway. She's had quite bad diabetes thats ever worsening, broken a hip all the classics basically. 50 years ago she wouldn't be alive in the same situation. She could well live to 90-95 which would be over average life expectancy, but she's in pain a lot with lots of ailments and so on. The age at which her body started to not function perfectly was probably a very similar age on average to what happened 50 years ago.
We're living longer with illnesses at the end of our lives, as opposed to living longer fully healthy before getting sick is what I'm getting at. Due to that the "age" at which we kind of look at people as "old" hasn't changed as its the kind of time things like joint pains, chronic illness's and so on start to kick in and to a certain degree thats when mobility decreases, ability to run around like a younger person is gone, needing help doing things and needing medical help. That age hasn't really changed.
Theres really only so long a body can keep patching itself up back to normal and we haven't changed that yet despite longer life expectancy. I can see life expectancy growing further still but that will still mostly be extending the amount of time we can survive with various illness's and not increasing our healthy lifespan.
Anyway, thats why old is seen as quite a specific period of time. Incidentally my gran was diagnosed with diabetes a couple years before retiring at 65 I think, and fell and broke her hip about 68.
I always thought in a just being a kid and looking at people further on in their lives that 30 was a pretty big milestone. One where a lot of people would be married off and have 1 or 2 kids and a secure job, I'm not there yet, but its not "too" far off and not seeming like I'll get married anytime in the near future and no kids(that i Know of) to mention yet so I'm starting to feel a tad old myself.