Measuring life/age in years is done for the same reason that any other unit of measure is created...because it allows for a point of reference to others. Running a mile might be a torturous excruciatingly long event for you, but a quick sprint through the park for others. OK, so how about we make it a subjective unit of measurement. That way, when I ask you how far down the road I need to go before I turn right, I also have to figure out if when you say a mile you mean 20 miles, or 2 blocks. Yeah, that sounds efficient.

So measuring a life in years is simply akin to that...it is a measurement required because the law is not capable of determining which people are mature enough at 15 to drink responsibly, drive and vote. Age is used in medicine as well, as a guideline because in general, a 2 yr old's body cannot handle the same medications/treatments that a 20 yr old's can. It is used to set an understanding of what world events someone has lived through, what generation they belong to, etc.

To equate age with quality of life is unreasonable. I'd far rather have 50 great years than 100 lousy ones. At the same time, I don't want only 40 good years because I'd not be around to see my children graduate or get married or have children.