This is off the Starting Strength website by Mark Rippetoe the author of an excellent book I've mentioned before, Practical Programming. I've posted twice about Rippetoe and his thinking - here's one Short read: Building strength; high reps with low weight v. actually building strength .
Here's the article. 15 paragraphs long, written with some of Rippetoe's sardonic wit. Takes about five minutes to read.
This is very important: stronger people live longer, no matter what the other variables might be. In studies of all-cause mortality, stronger people have a lower mortality rate at any age. Any profound loss of LBM (lean body mass) is highly correlated with mortality. Heavier people tend to be stronger. Heavier people carry more muscle mass, and more muscle mass correlates positively with strength, even if they are fatter.