If someone is 15 then any advice is meaningless as they are at least 5 years away from entering the job market and no-one can possibly predict how good or bad things will be that far down the line. There is no such thing anymore as a "cadetship".
As for those talking of training for an upturn, you are all admirable for your enthusiasm, but also complete fruitcakes.
I work in the industry, I've a fairly safe seat, for now. I'm not alone in believing things are going to get much worse. This time next year there will have been some big changes and a lot of people currently gainfully employed in big, well known airlines will be out of a job.
So for those of us with a little knowledge of the airline business, you can see how we view those hopefuls' talking of "training now for the upturn" as something of a tragic comedy.