Hi due to the on-going security issues and the costs of issuing airside ID and insurance, most if not all airlines will not accept any work placements.
If you are really wanting one of these placements then do keep trying.
Telephone the airlines and request to speak to the recruitment department or cabin crew managers.
They may just have you working in the offices but at least you got your foot in the door make a good impression and maybe get offered the job that you want when you are old enough.
Let us know the outcome.