Why Milestone?

You may have noticed that I received an invitation to check out a blog about turning 40 in the comments to the post "Feelin' It."


I don't exactly get it. Turning 40 is a milestone, but of what? Why? Because people say it is? Why do they say it is? I'm not seeing any answers on that blog yet. What would "experiencing Turning 40 to the fullest" look like?


At 16 you can legally drive (though I know that recently a lot of conditions have been put on 16 and 17 year old drivers).
At 18 you can vote, and if you're male, you must register for the Selective Service.
At 21, you can legally drink (but a lot of people who will drink after turning 21 started when they were well under 21).
At 50, you get an invitation to join AARP as a full-fledged member.
At 65, you qualify for Social Security payments and Medicare health insurance, and you can withdraw money from your IRA without big penalties.


I'm not seeing 40 in there as a significant marker, unless you count getting mammograms - and only 51% of us have to do that, and a lot of women start getting them before they turn 40. So why is it such a milestone?