I don't understand your question ("they"). Making it to 20 depends on your health and assignments and the decision of the soldier. Going beyond 20 depends on where you are in the pecking order with assignments, rank, and schools. I've got 4 NCO's over 22 going for 26. Most stick around to get another deployment.
I'm agonizing over the decision to stay or to pursue other interests (39 years old).........maybe a bottle of scotch and flipping a coin would the best method.