Found: YA Fantasy Book

I read this book maybe 5-10 years ago and only vaguely remember any details. From what I recall its about a teenage male who keeps getting letters in the mail from an unknown source. Eventually he finds out that a old man has been sending him and other kids letters in the mail calling them to save the world. I know its not a lot to go on but any information or thoughts of what it might could be would be amazing. Thanks!

Long shot, but maybe Mr. Was by Pete Hautman?

Probably not it, but reminds me somewhat of The Mysterious Benedict Society.

I found it! It was The 13th Reality by James Dashner. Thanks for the guesses though!

Touchstone: The 13th Reality