Keith Szarabajka mastered the gray look early at 14 he played Grandma in Edward Albee's The Sandbox at his Wisconsin boarding school. Now, says the 38-year-old actor, ''even my girlfriend (actress Jennifer Jay Stewart) likes me better as an old man,'' which is lucky. Szarabajka has gone geriatric again to star in Stephen King's Golden Years on CBS as Harlan Williams, a feisty 70- year-old janitor at a government lab who gets doused with a green goo that well, let's just say it puts Grecian Formula to shame. ''This is the best part I've had since I played a dog in Sirens of Titan at (Chicago's) Organic Theater,'' adds the Oak Park, Ill., native, the son of a teacher and a financial officer. Szarabajka (he's Irish, Polish, and Lithuanian) admits that in those days, college (at the University of Chicago) was ''a hobby'' he spent most of his time writing and performing with the likes of Joe Mantegna and John Heard. (His TV series debut was on CBS' The Equalizer.) Though not a true King fan (''I started reading Cujo once, and it was too scary''), he gladly submitted to the six-hour makeup job needed for his role. ''I haven't played an older part since high school. They hire me for these juvenile roles and I'm, like, wait a minute I don't understand all these young people!''