> Are you extending the healthy, active period, or just dragging out that bit at the end when someone else has to wipe your bottom and feed you apple sauce?

It's always both. Mortality isn't an independent counter, it's linked to a bunch of predictors that are also predictors of health and autonomy.

In other words, if you "eat the other half", you won't just die younger, you'll also start eating applesauce sooner.