I used to work in marketing and young was always the 28-34 demo. Browsing on line recently I was looking at a modern ad agency (forgot name) focused on digital influence, as opposed to traditional print and broadcast media. Their young demo was defined as 18-44. So I do believe that as we live longer, our period of youth/middle age etc. is shifted.
My own definition: 13-24 is Teen/Youth. not having a fully formed brain yet, which happens at age 25 (source: NPR's Science Friday).
25-44: Young adult, with adult brain.
45-64: Middle age.
65 plus: Old age. As defined by the government, so, when we get Medicare. Social Security defines my old age as 67-1/2 (they've caught on that we're living longer).
I actually still thought of my own Mother as being middle aged until she was around 75. Middle aged is fully functioning mentally and physically strong. I do think that there are those who are not, physically or mentally, really old, until they are 80. Maybe even older. But there are plenty of people who are in decline by the age of 65. An acquiantance of mine is a few years older than I am, but having worked construction and in gas (propane), he has many more health problems. Also, his wife is in worse health than I am -- missing teeth, back problems -- enough problems to collect a disability check. No one has mentioned this in this thread, but socio-economic factors do play into how we age.
As for myself, I am interested in staying in good health and being strong. When I was in my 40s (I'm currently 57), I tried to hold on to youth a couple years longer by doing things like botox. Now I hold no illusions of passing as a younger person than I am. But, I want to be strong and healthy and my aesthetic aim is to be stylish (not necessarily to look younger). So that means I have wrinkles and sun damage and people correctly guess my age. (The wrinkles are also connected to having lower BMI. So there's a tradeoff.) Of course if I routinely passed for younger I would probably love it too. Since I don't, I've simply learned to embrace myself at my age in my current condition.
Thanks Angie for starting this amazing conversation.