Why Measuring Ages In Numerical Decades Is Stupid
We often refer to stages of our lifetimes by the decade, e.g. your thirties, your twenties. This actually makes very little practical sense. See below.
1. Age 0-10
From a new-born baby to a pre-teen. Some girls will even have puberty effects by 10. There's very little common thread for that decade, but that's OK, as children are developing and it's a constant change.
2. Age 10-20 (your "tens")
Imagine a 10-year-old, then imagine a 20-year-old. The former might still play with toys, and the latter could be sexually active, vote, work a job, or die for their country.
A lot of people in their early 20s will be finishing an undergraduate education, and if a graduate degree or program. They might be eating ramen to get by with their tuition and student debt weighing them down. Individuals in their late 20s might have an established career and good income, or maybe they're up to their eyeballs in start-up/entrepreneurship culture.
4. Alternate proposal
I could break down every decade like this, but as I'm at the tail end of my forties, I'll soon run out of my own life experience with which to measure these decades. Instead, let me propose decades at the halfway mark.
The cerebral cortex is not considered fully developed until 24. I believe it's difficult (or even impossible to rent a car in some jurisdictions before 25. While I'd never recommend it, there are examples of 15-year-olds being the sole or prime providers of their households (not in societies where child labour laws get in the way, more in a historical sense or in developing nations). A 15-year-old is closer developmentally and demographically to a 25-year-old than a 10-year-old to a 20-year-old.
This grouping will generally be about getting established and settling down. They're sophisticated enough to know what they want out of life (mate, career, home) and start seeking it out. When they reach their target will vary, but it will happen within this decade. A 25-year-old is more similar to a 35-year-old than a 20-year-old to a 30-year-old.
Starting a family. For those that don't go down this path, they will start being able to use the disposable income to pursue or even master their passions (e.g. becoming a pilot, scuba diving, travel, music, etc.). I became a father at age 35 and completed a half-Ironman at age 42 and yet the man that had the energy to do both those things seems all but dead to me now at 48 (pushing 49). Somewhere after 45 there was an acceleration of physical breakdowns and age really started to show. It's possible that hormonal changes occur too and affect my moods. 48-year-old me is completely different than 40-year-old me, but 34-year-old me and 44-year-old me feel like they would be largely on the same page.
8. Looking ahead
Grouping an age like "Your Sixties" also seems wrong. The first half would be still occupied by work (till the nominal retirement age of 65) while the latter half would be figuring out how to stay busy in retirement. 65-75 would be all retirement, and 55-65 is a "late career" decade. Individual mileage will of course vary, and much of this is based on Western, first-world society, but consider: 15-25 (Young Adulthood), 25-35 (Establishment Adulthood), 35-45 (Settled Adulthood), 45-55 (Middle Age), 55-65 (Late Career), 65-75 (Retirement)