On the contrary, active life expectancy is increasing faster than life expectancy, according to the experts I interviewed for an article in the Atlantic. You can see this in the proportion of people on Medicare who have limitations in their ability to perform the tasks of daily living, which has been falling, not rising. (Note: this chart is from 2008, before the bulge of baby boomers started skewing Medicare demographics younger).(links omitted, available at original)
So yeah, we may be getting more fat, but it doesn't seem to be making us die earlier or have less active, less healthy elderly years--to the contrary, in fact.