if he were a mutant the whole time... he could be potentially a very old mutant (an could have been around for decades even...) at the point that we see him as Dr. Schmidt's he may of not had used with powers in a while (that rejuvenates him, and keeps him young) and let himself naturally age abit...
but, throw coming across young Erik, and realizing a new generation of mutants have come... he starts to plan his raise, and takes on the new identity of Shaw, to put it in to play... an spends 15 years or so, rebuilding his empire, and recruiting others to his cause (to a guy potentially as old as him, 15-20, is nothing, in the grand scheme of things)