Elena Kagan is 50 years old which is not much younger than the average age of newly appointed justices: 53. That average age upon entry has been relatively constant over time but with life expectancies steadily increasing, the average tenure on the court has increased from 15 to 25 years before and after 1970.
We could argue about the socially efficient entry age and tenure length but its more fun to think about strategy. As a President from The Democratic Party you are today’s player in the infinite-horizon alternating-move SCOTUS appointment game. It is essentially a game of tug-of-war: they will appoint conservatives to balance out the liberals that you will appoint in order to balance out their conservatives…
The younger your appointee the longer she will sit on the court. On the plus side this means she is less likely to die or retire early. On the down side you will have to live longer with a Justice whose views are harder to discern and are more likely to change.
Tradeoff? Less than it appears. It boils down to a comparison of two probabilities: the probability that the older Justice will step down in a year when the Republicans control the White House versus the probability that the younger Justice will switch teams. Unless there is a lot of uncertainty about the younger Justice, the second probability is smaller and you should appoint her.
How young should you go? As you consider younger and younger nominees the mid-tenure defection eventually becomes the dominant concern. The probability that a non-defector can retire under a Democrat administration reaches its maximum but the uncertainty surrounding a younger Justice steadily increases.