What is the penalty for overstaying a tourist visa? How long do I have to wait before applying for visa to get back to the US?

Based on the description you provide, the penalty that I believe you have incurred is a three year bar from returning to the United States. By overstaying your visitor status, you accrued unlawful presence. However, as a technical matter, you only accrued unlawful presence once you turned 18. Any unlawful presence before your 18th birthday does not attach to you as a penalty for returning. If a person has more than 180 days of unlawful presence, but less than 1 year, the penalty for returning to the United States is 3 years. Your parents, however, accrued more than 1 year of unlawful presence, and are therefore prohibited from returning for 10 years. There is a waiver available under INA § 212(d)(3), but with such a long period of having overstayed your status, the Department of Homeland Security is unlikely, in my opinion, to approve such a waiver.