Supreme Court justices are often choosen not solely based on their professional skills and reputation, but also based on their political orientation in order to advance certain ideology. One example is Kavanaugh.
The length of the period for which such judges will make rulings based on that ideology (if they don't change their own ideology themselves) is limited by their lifetime and health. Therefore, if politicans want to advance their ideology for as long as possible, they might want to appoint a very young and healthy person as a judge (say, a 22 y.o. person who has just finished bachelor in law progam). Yet, the youngest US Supreme Court at the moment of appointment justice was 32 and only 4 of justices were less than 40. So, why no younger idelogically-motivated appointments?
Note 1: the question is mostly US specific, but any references to other democratic countries with lifetime supreme justices are welcome.
Note 2: please keep in mind that the question is not why all justices are such. Having a Supreme Court consisting of 9 inexperienced undergrads doesn't sound good, but having a Supreme Court consisting of 8 experienced professionals and 1 young person having similar ideology to yours sounds like a good bargain if you believe that skewing Supreme Court towards your ideology is a right thing.