That is an interesting question.
I go for the health organizations' definitions.
0 to12 months - baby, 1 to 3 years - toddler, 3 to 5 years - preschooler, 5 to 12 years - gradeschooler. I consider these as younglings. When all worries you have are solved with a quick cry and you're untouchable by the law.
12 to 18 - teen, 18 to 21 - young adult. I consider these something to come before the middle. The best time to discover the world.
22 to 40 - adult age. Gues it's the middle ground. The best time to put your world discoveries to work.
40 to 60 - young elder. The middle between adult and old. The preparation for retirement.
60> - elder age. When we just can't do much anymore.
I base my opinion on 40s to 60s being a middleground between adult and old because we almost completely lost the youth we had when teens, configuring "old age", but are still up and kicking.
And I find a necessity to exist a middle ground between young and adult, because we don't become full adults in an instant.
World Health Organization