For a civil suit, you would be too late since the limit is one year after your 18th birthday.
But more importantly, you can talk to the police and pursue criminal charges. There is NO statute of limitations for criminal sexual conduct in the first degree. For criminal sexual conduct in the second to fourth degree, and assault with intent to commit criminal sexual conduct, the case must be filed file within 10 years after the offense is committed or victim’s twenty-first birthday, whichever is later. All other indictments may be found and filed within 6 years after the offense is committed.
I would strongly suggest discussing the matter with the poilice. Due to the long term nature of this abuse, I would expect that your step father would be looking for another victim now that you have gotten old enough to get away from him. Just go to the police station in the city/county where the abuse in occurred and tell them your story.