The main issue with Keystone masteries is that they provide a significant boost in damage across the board even without items.
That is a large part of the reason we have had so many assassins building defensive items this season (i.e. Akali, Yasuo,). Keystone masteries provide enough additional damage they they can still kill squishy champions without itemizing for offense and they need the additional survivability to survive against all the damage being thrown around.
Strength of Ages is perfectly fine for junglers. It stacks up at a reasonable rate, 300 HP is massive in the mid-game and it provides useful sustain in the late-game (cannons dying during sieges, restoring health them killing jungle camps).