Okay, the tally so far (hope I didn't miss any):
1973
1984
1973
1966
1974
2001
1996
1964
1977
1979
1964
1961
1973
1955
1993
1967
1969
1978
1978
1956
1989
1976
1960
1969
1991
1991
1984
1976
1964
1992
1961
--------
1975.290323 (avg)
So, the average year is ~1975. Given that most were probably in their teens when the deed happened, that either means we have some old farts here, or people doing it in cars at least 10 years old at the time, or like me, both...(I'm 41 and my car at the time was 11 years old).
TJR