What would be a good time for a 15 year old?

Kick Sprint 45
I have some advice for you
First you might take a look at the sites for some of the following people, Ben Johnson, Usain Bolt, Lindford Christie or Donavan Bailey.
I am guessing each of them will have hoards of free information for you and even design a custom training protocol so that you can understand or calculate in 24 hours or less the art of conditioning, speed and strength.
But before you do that I will direct you to the expert feedback you just received above from this forum via CharlieFrancis.com which I assure you is long from dead in anyway shape or form. ( no pun intended son)
BTW
you might want to invest some of your paper route money or stock interests
and buy SpeedTrap an ebook for sale in our store available instantly which sounds like it would appeal to you.
This would be my advice on your starting point.
Bon Chance!