No, you don't have to wait. I think what they are asking about is cases where someone was abused/raped when under 18 but for some reason was unable to prosecute right after it happened. There are all sort of reasons why this might occur. For instance a 12 year old girl may be to ashamed to tell anyone that someone was abusing her. Sometimes the victim is threatened that they will be harmed more if they tell, or is convinced by the perpetrator that no one will believe them.
I believe you can press charges, but there may be some restrictions on time frame. I think that is how all those allegations against priests came out. I don't know how many were able to prosecute as many of those victims did not come forward for many years due to shame, fear ect.