I Found Swordsman Kid today , i think only adults can be a master class then i forgot about adults after seeing this! but how to do this?

You can become a kid with your masterclass by using a kid clock. You can buy them from the Auction house (AH) or by using cashpoints and drawing them out of the wagu wagu machine.

Please note that once you use a kid clock, the only way to become an adult again is by using another clock.

Here is a screenshot of the item: