In medical culture the life stages are as follows:
0ish = neonate
0-17 = paediatric
18-49 = very young
49-79 = young
80+ = old, but it also depends
I like that so I'm planning to be young until I'm at least 79.
Realistically, middle age is a little different for everyone but in my mind begins somewhere between 40-45 and becomes old age as early as 60 but often not until 70ish. The end of middle age / beginning of old is the time when even people who find themselves in good health just start to feel old and tired, and can't manage their old responsibilities as they used to. Entering middle age, I think has mostly to do with leaving behind a youthful appearance and also having left one's reproductive prime. And also relates to the middle of adult life (not the middle of the whole life), which doesn't begin until around 18-21 these days.