At the risk of stating the obvious, did they have a look and made sure there were not oodles of wax in your ears?
Beyond that go and see your GP so they can have a look. If it was due to a cold (not uncommon) then you may well be OK on a repeat test.
If not it may be worthwhile to go and see an ENT surgeon to ensure there is nothing wrong further up the line. A disqualifying hearing loss for a youngster is unusual and warrants a closer look.
Not sure what you mean by a redress of the situation?
Would have thought that they would struggle not to take you on if the hearing could be rectified with a hearing aid etc. They can probably justify that you would a reasonable amount of hearing to do the job as flight cabins are a fairly noisy environment.
They don't just want you to be able to hear what the punters and the tannoy says, they also have a duty to protect you from your hearing loss getting worse.
Two sides to the coin, as ever.
Best of luck.
FD