YOUNG POLITICS: Not everybody in Orange County's delegation to the Republican National Convention this week is a party veteran. Chad McWhinney of Huntington Beach hasn't even reached the legal drinking age. . . . But the 20-year-old alternate delegate has been making his mark: At 18 he was appointed to the state Republican Central Committee. . . . It's all in the family. His father, Derek, was elected mayor of Westminster in 1969 at age 27--youngest in California.