It's not the years, honey, it's the mileage.

My Dad is 80 today. That's nothing. Ben's friend Luke has an 82 year old grandfather who is getting married this weekend.

LONDON (Reuters) - Is 40 really the new 30? In many ways people today act younger than their parents did at the same age.

Scientists have defined a new age concept and believe it could explain why populations are aging, but at the same time seem to be getting younger.

Instead of measuring aging by how long people have lived, the scientists have factored in how many more years people can probably still look forward to.

"Using that measure, the average person can get younger in the sense that he or she can have even more years to live as time goes on," said Warren Sanderson, of the University of New York in Stony Brook.

He and Sergei Scherbov, of the Vienna Institute of Demography at the Austrian Academy of Sciences, have used their method to estimate how the proportion of elderly people in Germany, Japan and the United States will change in the future.

The average German was 39.9 years old in 2000 and could plan to live for another 39.2 years, according to research reported in the journal Nature on Wednesday.

However, by 2050 the average German will be 51.9 years old and will be expected to live another 37.1 years. So middle age in 2050 would occur at around 52 years instead of 40 years as in 2000.

My head hurts.