Thanks Fleet!!!A betta is usually around six months to a year old in a petstore, but don't worry! They can live up to five years with proper care. You might see some red wash as he ages, but it's okay.
Marbelling is a type of gene, not any indication that Blue will die. It's one of my personal favourite colours
You can read up on marbelling on Betty Splendens here:
http://bettysplendens.com/articles/page.imp?articleid=973