Some aging brains stay sharp - but why?

It's always amazed me to see some older folks I know seem as mental sharp as a tack while other fade into mental forgetfulness at the same age. Why is this? It seems as though the brains of some shift strategies as age increases in order to keep that mind alert and in focus.

Scientists are wanting to determine what differences these aging, sharp minds have in order to possibly leverage what they find into a method to keep all aging minds on the sharp side.

The scientists wanting to make a discovery here say that this has nothing to do with diseases like Alzheimers, but ensuring the mental faculties of a longer-living public stay with them along with their physical age. Nobody would want to live 30 years without his or her mind at close to full function, right?