There are no age restrictions as far as I know.
The benefit is that it's the most widely known measure of Japanese language proficiency and gives you a goal towards which to study. It helps structure your studies.
The format changed as of this year so make sure any texts or mock exams you practice with are for the new format, N1 - N5 (not JLPT1 - JLPT4)
One of the benefits of this new format is that it is a better measure of your ability to function in Japanese vs. simply memorize vast amounts of information. The reading/listening process has been integrated so that you must listen/read/comprehend simultaneously; listening to a conversation about 4 newspaper ads for example. It's less 'testy' and more functional.
This is similar to English TOEIC, and also the JETRO business Japanese test, which already used this format.
My humble opinion is that if you're a fan of Japanese and studying as you seem to be, it's fun and educational and taking the test itself is enjoyable. Take it this December and see if you like it. Costs a teeny bit of money but so does everything else in the world.
Good luck