A 2007 report by the Iowa Department of Corrections puts the recidivism of a sex offender who receives "some treatment" at 1.8%. But this study does not separate the released inmates by age. The 1997 Federal Study by the United States Bureau of Justice Statistics (A part of the United Stated Department of Justice) states that sex offenders older than 45 have only half the recidivism of younger offenders. So taking that 1.8% for all sex offenders released in Iowa and cutting in half gives me a rate of 0.9%
It may be even lower as heterosexual incest offenders have the lowest recidivism of any group of sex offenders.