;; has-teen-or-child?:  person -> boolean
;; consumes a person and returns true if anyone in the person's tree
;; was born after 1991
(define (has-teen-or-child? a-per)
  (or (> (person-year a-per) 1991)
      (youngster-in-list? (person-children a-per))))

;; youngster-in-list?:  list-of-person -> boolean
;; returns true if anyone in list, or any of their descendants, is
;; born after 1991
(define (youngster-in-list? alop)
  (cond [(empty? alop)  false]
        [(cons? alop) (or (has-teen-or-child? (first alop))
                          (youngster-in-list? (rest alop)))]))