Hi Andre. As I say to all people who risk using their real name (if this is your case) be careful what you say. Joining at 17.5 years old requires a certified authorization from your parents or guardians.
We had another guy form Ireland (Williams) who wanted the same thing. I'd put him in touch with the president of the AALE Ireland whom he never contacted. The President, a very good friend of mine, told me to pass on that he should contact the French Embassy in Dublin. My friend is very close with the embassy and said that he never got in touch with them.
I then wrote to the recruiting centre in FdeN to ask what the requirements to join at 17.5, but never got a response.
The only thing I can suggest is that you go to the French embassy or nearest consulate and ask them the formalities required for joining the LE before 18.
Or failing that, get your letter and then have it translated into French by an authorized translating office and try your luck.