Aside from being an important trope - the villain needs to be a threat but also then overcome by the heroes - there is also the fact that all of these things are events that are guided by the force.
Remember - the force is not just a weapon or a tool, and the light and dark sides of the force are merely that, two sides of the same coin.
Connection with the force tends to grow stronger with age as the person matures and grows the patience to study and meditate upon the force and so allow it to shape their thinking, guide their actions and decisions. This is shown well in the way that we see Obi Wan develop over the years, from padawan to master to Old Ben.
The force isn't bias, it does not favour either jedi or sith, but rather it allows them both to prosper and grown and then be culled in order for the opposite side to flourish for a while because all things have their time, but that time is never forever.
Eventually, all things must come to an end and all force users must die, as is the will of the force, whether the wielder knows it or not. Some of the more powerful users have an inkling of when it is their time to go, such as Ben and Yoda.
In the instance of Darth Maul, he was a far better fighter than force user, as was shown in his approach to combat. Someone like Dooku however was simply overpowered by a far stronger force user, not because the jedi was younger but because the jedi was simply stronger and in his youth more physically in shape.
