One of the charge accounts at Buffum's Inc. in Long Beach, Calif., belongs to a female who can't talk, read, try on clothes, or sign checks. At least not yet. She is only one year old, the youngest member so far to qualify for Buffum's Premier Credit service, available to customers 17 years old and under. In the first year, says Gordon Eatherly, corporate credit manager at the 15-outlet specialty department store, more than 2,500 children (backed by parents with good credit histories) have been OK'd for credit limits of up to $200. Is the one-year-old a heavy spender? "Right now," Eatherly admits, "there's not a whole lot of activity on that particular account."