At first, children find it difficult to distinguish words and other mental symbols from the objects and actions to which they refer.
( ( At first ) ( , ( children ( ( find ( it ( difficult ( to ( ( distinguish ( ( words and ) ( other ( mental symbols ) ) ) ) ( from ( ( the ( ( objects and ) actions ) ) ( ( to which ) ( they refer ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (JJ first))) (, ,) (NP (NNS children)) (VP (VBP find) (S (NP (PRP it)) (ADJP (JJ difficult) (S (VP (TO to) (VP (VB distinguish) (NP (NP (NNS words)) (CC and) (NP (JJ other) (JJ mental) (NNS symbols))) (PP (IN from) (NP (NP (DT the) (NNS objects) (CC and) (NNS actions)) (SBAR (WHPP (TO to) (WHNP (WDT which))) (S (NP (PRP they)) (VP (VBP refer)))))))))))) (. .)))
Once children reach adolescence they no longer have problems differentiating words from objects. 
( ( Once ( children ( reach adolescence ) ) ) ( they ( ( no longer ) ( ( have ( problems ( ( differentiating words ) ( from objects ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Once) (S (NP (NNS children)) (VP (VBP reach) (ADJP (JJ adolescence))))) (NP (PRP they)) (ADVP (RB no) (RB longer)) (VP (VBP have) (NP (NP (NNS problems)) (VP (VBG differentiating) (NP (NNS words)) (PP (IN from) (NP (NNS objects)))))) (. .)))