I've never really thought to define it, I may use the term occasionally to indicate I'd been speaking with someone who was neither very young or very old.
Well my 86 year old mother won't live in sheltered housing because it is full of old people....... I feel for women it's when you wake up one day and know what to do with those long scarves and for men it's when they consider a slipover sleeveless jumper a nice addition cos it keeps your back warm
given that youth seems to extend to about 40 these days, I suppose middle age starts then and goes to about 90. Everyone seems to claim eternal youth. But I can't say I feel in the least middle aged, I'm old.