The character of Helen Hanson/Teresa Hamilton is American, but the actress who plays her, Michelle Ryan, was born in England. In this episode, she gets to use her native accent, rather than her affected American accent.
The story narrative would seem to strongly imply that Helen Hanson / Teresa Hamilton (Michelle Ryan) has been in the spy game far longer than Annie (Piper Perabo), by well in excess of at least three years. Previous episodes have indicated that Annie engaged in other life experiences prior to joining the CIA, so, the relative ages of the characters are hard to determine. However, in real-life, Piper Perabo is approximately 6.5 years older than Michelle Ryan.