Like surfers carried to the shore, these stars were caught in the spiral waves of our swirling galaxy, which carried them outward and dumped them in the outer regions of the Milky Way. Naturally, it takes longer for them to travel farther, so the outermost stars are also the oldest.

Funny, that's not what the graph is showing, by my interpretation. To me or looks like the innermost stars are the oldest. Then the mid stars are the youngest, then outermost are older again but not the oldest.