hit on a 15-year old 
(ROOT (S (VP (VBD hit) (PP (IN on) (NP (DT a) (ADJP (NML (CD 15) (HYPH -) (NN year)) (JJ old)))))))
(ROOT (S (VP (VBD ) (PP (IN ) (NP (DT ) (ADJP (NML (CD ) (HYPH ) (NN )) (JJ )))))))