You can't. 14 years old is too young to work, and game testers are usually employed by the licenser.
I know what u are going through looking for this kind of job im a 14 year old boy and been looking for a beta testers job for like 20 days and i found a gamers site it helps you get a beta testers job testing the newest games for xbox 360 Nintendo gameboy etc. it said 15 but i checked it after and it said i was qualified so im goin to check it out and il tell you if its worth looking into. best of luck to you.if u want to see this site send an email to email@example.com il send the link and any and all info i know about it and/or some other beta testing sites