Gym masters are dedicated to train their Pokemon and and are very familiar with their environment. They should have a vast advantage over trainers, especially younger ones.
If that is the case, why don't they defeat more trainers? Shouldnt only trainers with comparable experience defeat them? Ash can defeat much older masters who have been training for decades. Is it just to advance the story or is there a rational explanation?
They are not easy to beat, Ash isn't just your average trainer. He might seem stupid/have really weak pokemon sometimes, but he is kind of world class when you look at some of his greatest accomplishments. And many gym leaders do beat him, and he eventually comes back to win. Gym masters do beat most challengers, just beating all of a region's gyms is qualification enough to take on the Elite four.
Also, gym masters are normally trainers who work within the rules of the gym, a fighting master tends to mainly have fighting pokemon, so will always be at a disadvantage to a trainer who wanders in with a bunch of psychic/flying pokemon. They are never portrayed as invincible or easy, just a bunch of tough trainers, well above the average guy wandering around with a bunch of pokemon.
It's a really good question, but hey, in the anime and in the games, we are not talking about normal trainers - we are talking about passionate and strong trainers. They are that elite which groups like Team Rocket etc fear.
But to come back to your question:
They should have a vast advantage over trainers, especially younger ones.
They do have an advantage - they are, as you said, trainers, which are familiar with their environment. But that doesn't mean they are impossible to beat. Also, their age shouldn't play a big role in winning or loosing a battle. I believe that battling with Pokemon is like playing a video game - when you start with it you can be good or bad. The more you do it, the more experience you get, but even a guy who plays a game for 5 years can be beaten by a rookie, simply because he feels how to do it right.
To come to a conclusion: I believe that gym leaders are strong trainers but the trainers we know are on another level than the gym leaders.
In Pokemon Origins, it is strongly implied that gym leaders own several pokemons of varying strength.
Brock is shown to have many Pokémon, and selects Geodude and Onix when Red confirms that he has no Badges. This implies that all Gym Leaders have Pokémon of varying strengths in reserve in order to keep each challenge a fair fight.
Source: http://bulbapedia.bulbagarden.net/wiki/Brock#In_Pok.C3.A9mon_Origins
So basically, gym leaders handicap themselves to get down to the trainers' level, depending on the number of badges the trainer has.
