I'm not 100% sure, but John Isner--at 32, almost 33--may be the oldest first time big title winner in Open Era history, or at least since the very early years. I'll have to do some research.
OK, found three first-timers who were older:
*Marty Riessen was 33 yrs, 1 mo when he won the 1975 Philadelphia WCT.
*Mark Cox was 33 yrs, 4o when he won 1976 Stockholm.
*Andres Gimeno was 34 yrs old when he won the 1972 French Open, but his inclusion is questionable as he won bunch of titles before the Open Era, at least one of which was presumably Masters level or better. But who knows.
If you're wondering, Ivan Ljubicic was 30, almost 31, when he won the 2010 Indian Wells.