Ex-PC Colin Hart, 61, from Nelson, Caerphilly county, has been charged with seven counts of indecent assault and one of raping a girl under 16.
He has denied the offences, alleged to have happened between 1988 and 1990.
Cardiff Crown Court heard he allegedly started assaulting the girl from the age of 12.
But Mr Hart claimed she was "at least 17" when a sexual relationship began in 1992.
The jury in the trial of an ex-police officer accused of raping a schoolgirl has retired to consider its verdict.