New York Herald Tribune!
In a film staged to the French student revolutions of 1968, if Isabelle's first words were "New York Herald Tribune!" from "À bout de souffle" in 1960 -- then by logic as an 18-20 year old, her first words were when she was 10-12 years old that she first spoke.
UPDATE: It's stranger, she states that she was born in 1959. That would make her 9 years old *during the events* and 1 year old when that film came out. On the other hand, Matthew stated that he was 20 and they were the same age. Unless he was trying to show the twins as both stuck at the same age in duplicate (9+9=18), then it doesn't make sense.