Karl Miller:

>Is there any composer today who, at the age of 40 or so, is considered
>a master, with each new work of his or hers being eagerly awaited by the
>concert going public?

Not sure how eagerly, but perhaps:

John Adams
Jennifer Higdon
Theofanides
Poul Ruders, at least in Europe
Ades (?) in some quarters;  I can't stand his stuff myself

Jim Tobin