How to start a fashion career young?

[aha!]

I can't say my advice is comprehensive, but I recommend the following:
interning at a fashion company (although I heard they are getting a bad rep for it)
have a portfolio (I am not sure whether you should publish your work online though); this would include samples of your outfits you made, with models wearing them (the models can be your friends; they don't need to come from an agency)
follow those stylists you admire online via social media e.g. blogs, twitter, etc.
have a scrapbook, or mood board, for inspirations in your design, and as a way of keeping yourself up to date with what is going on in the industry
do a lot of networking with people who are already in the industry, and those in fashion schools

Hope this helps