How much does age affect the powers of Jedi and Sith?

1. Canon.
2. Legends.

2. It shouldn't affect at all. It depends on personal use.

1. Stronger.
2. Depends. Stamina/physicals decline, power usually increases.

With the emphasis being on power, the answer is they grow stronger in both.

they grow stronger

Age doesn't effect your powers, but physically it does.

It seems to affect them quite a lot, but the losses suffered are often compensated for with the skill and experience that develop over time.

It definitely affected Qu-Gon Jinn.

They grow in the connection and knowledge of the force, even if they decline physically, the augmentation they can push on themselves makes them more dangerous, the best example of this is Yoda who has arthritis, depression, and trouble breathing. Yet Yoda flies into battle like a flying green discus faster than almost anyone in the verse, this is due to the power he possesses being enough to circumvent his physical handicaps. The only thing age does in spades though (seen with Yoda, Dooku, and Sidious) is limit the amount of stamina they can have and when thier own augmentation and power runs out they are left as little more than helpless old men.