1. Yes, but it starts from the first, second, etc.

2. Nope, pre-60s benefit from it too

Summary of Pools of Reflection:

- You get a 25% bonus XP gain which stacks with your overall XP bonuses (multiplicative)
- Per stack is 10% of the total XP needed to gain for the next lvl (or plvl) when you touched it e.g. total XP to next level = 1,000,000, Pool's bonus is 100,000
- Additional XP gained from the 25% bonus XP while under the influence of the Pool of Radiance regardless of stacks will deduct the overall bonus XP left until 0

It's random and sometimes they appear more than you need. Max stack of Pool of Reflection bonus is at 10.