Question: when was the voting age changed to 18
[{'answer': 'July 1 , 1971', 'provenance': [{'wikipedia_id': '70136', 'title': 'Twenty-sixth Amendment to the United States Constitution', 'start_paragraph_id': 24, 'start_character': 181, 'end_paragraph_id': 24, 'end_character': 194, 'bleu_score': 1.0, 'section': 'Section::::Proposal and ratification.:Ratification by the states.\n'}]}]