I think there are two reasons: because we are living longer and because it's become socially acceptable for people to dress and behave as if they're young for longer.
My mum was 31 when she had me and would be 85 if she was still alive. I can't ever remember her not dressing "middle-aged." When I was very little, and she can't have been more than about 36, she was wearing those scarves tied under the chin like the Queen. (She did give those up in the end!) She never owned jeans and only wore trousers when she was doing something practical that in her eyes demanded them. She never had long hair in my lifetime and thought she was too old for it. In old age she wouldn't wear make-up because she thought it was somehow inappropriate or tarty on an old lady. OK, there are women her age who dress very fashionably or eccentrically, but they are still rare enough that people talk about how splendid and refreshing they are. She also led a rather conservative-with-a-small-c lifestyle - for example it would never have occurred to her to get drunk, and she would have been shocked if anybody did, but especially if they were older and "ought to know better."
Nowadays it is perfectly acceptable for someone of my age (50s) to wear jeans, have long hair and/or dye it unusual colours, wear shorts and strappy tops in summer, post on social media about how they "need" a gin or how they had a "cheeky" few drinks of prosecco with their female friends. It's a different way of life, and it contains elements that would traditionally have been associated with young people, so I'm not surprised the definition of "young" is creeping up.
FWIW, at 54 I would definitely describe myself as middle-aged, and I intend to describe myself as old at 60, but I'm very glad the unwritten rules for being middle-aged and old are not as strict and joyless as they used to be!