Leagues:
You can be any age. Just show up, and ask the person in charge of the league to give you a quick rundown of their league (the people, the preferred playing methods, and so on). Leagues are pretty casual compared to tournaments, at least where I am.
Tournaments:
You can be any age to enter, as they group you by age groups based on your age. At 15, you'd be in your last year of eligibility for the Seniors age group (10/11-15). You would need a POP ID, though, which you can get when you sign up.
Also, you stick with Pokemon cards you like or think work well together for a deck. Use actual cards, though, or you'll get banned from Organized Play for life. The number of cards you MUST have is 60. No more, no less. Also, build your deck before you show up to tournaments, as there won't be enough time after registration to build one.
Visit the official Pokemon website's TCG and Organized Play sections for more information. Come back here if there's anything there that confuses you.
