Why did Ash became a child back in Sun Moon Series? I was watching Pokemon Sun Moon and I think its a continuation because he knows about Pokemon from other regions.

But the question is that why did he became a small kid, from a young guy, who challenged Alain in Kalos League Finals?

He nearly won the match so why did he need to go to Pokemon School?
Ash Ketchum has, and will always be, 10 years of age. Sun and Moon has only changed the art style and focus of the anime. Whilst Pokemon of old had some comedy in it, Sun and Moon has made it a priority.

source for age: http://bulbapedia.bulbagarden.net/wiki/Ash_Ketchum