According to a Freedom of Information Act reply from the U.S. Bureau of Transportation Statistics (BTS), the last known pre-9/11 flights for three of the four aircraft involved in the terrorist attacks of September 11, 2001 took place in December, 2000, nine months before the attacks, while no pre-9/11 final flight information was provided for American Airlines flight 77 (N644AA).
However, a discovered searchable online BTS database produces the following search results for three of the four 9/11 aircraft on September 10, 2001:
AA 11 departs San Francisco (SFO): AA 09/10/2001 0198 (flight number) N334AA (tail number) BOS (destination) 22:04 (wheels-off time)
UA 175 departs San Francisco (SFO): UA 09/10/2001 0170 (flight number) N612UA (tail number) BOS (destination) 13:44 (wheels-off time)
UA 93 departs San Francisco (SFO): UA 09/10/2001 0078 (flight number) N591UA (tail number) EWR (destination) 23:15 (wheels-off time)
UA 175 and UA 93