BIG mistakes, both.
Congress passed a law lowering the voting age to 18 in federal elections. That would have created a headache for the states if citizens older than 18 but not yet 21 could vote in federal elections but not in state or local elections, so Congress quickly passed an amendment lowering the voting age to 18--it was approved by Congress on March 23, 1971, and was ratified by 3/4 of the states by July 1, 1971, becoming the 26th amendment. The President has no role in a constitutional amendment.