Age is not a problem, but the level of maturity is.
15 can make the job done, I think you should wait and spend more time on wonderland.tf, knowing other members and showing activity.
Additionally, people with low/negative ratings(the bar with the green, gray and red colors), misconduct on the forums/Discord or In-game, player disrespect (specially Staff members) can severely affect their application.
Racism or Staff disrespect=Instant rejection
Not meeting requirements with special marks (*) can also result in a rejection (Declined)
Best of Luck!