In general, the rate is very low. This is very problematic in general and may be by itself a reason for a reform.
Remark, what it is a very difficult question to get a correct estimate. It also depends very much on the general climate in the society.
Remark also that these rates may be different for consensual and coercive experiences. In the case of a consensual relation, there is no victim which is interested in persecution (even if it is usual to name the minor as "victim").
One possibility is to compare criminal statistics with the results of surveys.