I seem to recall the Bible saying in essence- God will grant anything you ask- so long as you ask for the right thing. As the parent of two daughters age ten and twelve- I could well tell them the same thing.
If you hash it out, it can kinda even makes sense. Then again not.
It can't hurt to ask. The differences between the three groups in the case study above are probably statistical noise.
I think God will do the right thing whether we ask Him or not. It's hard to imagine that He'd say, "I would have cured that stomach cancer but the group came up three prayers short."
Plus, I stress again, it's not important how long you live. It's how you live.