Many of them are in their twenties and have been training for years. Flint, for example, said he'd been training since he was a kid and he and Volkner are now twenty.Yes you would think that someone like Charizard or Pikachu or anyone else in his Kanto team that have been with him for many years could defeat some one like the champion. And plus Ash has defeated many legendary pokemon over the years.
Volkner and Flint were more or less equals until they reached age 20. Volkner stayed at the gym to train and Flint went out into many "harsh terrain" to train which is why Flint grew to be stronger than Volkner. Ash is like Flint, he travels while training and experiences much more than someone who stays in a gym.
Sense Ash is still supposed to be 10 and he's about to head to his 5th major region, who knows how many regions Flint traveled through training before he finally became that strong. He may have had to travel through 50 regions for all we know. Aging is obvious an indication of nothing. The official site says Ash is still 10, despite what others will argue about him being 12, 14, or even 16 now.
By the time there series finally ends, I'm sure each and every one of Ash's pokemon will look as unstoppable as Cynthia's Garchomp or Flint's Infernape. Charizard, Torterra, Sceptile, Gible, Buizel, Butterfree, Pidgeot, Glalie, Bulbasaur, Totodile,Pikachu, everything.