That depends on your daughter, but 9 sounds way too young. I was adopted as an infant & my parents always said when I was 18 they would help me if I wanted to. You have the right/obligation to protect your child from what sounds like might be a bad situation unless/until they are no longer involved with drugs etc. as well as to dictate when & how any introduction will happen while she is still a minor. She may not like it, but I think "I'm sorry, but it is not a good time for you to meet your bio parents, I will be happy to be there for you when you're 18 (or some other age) if you still want to meet them then." is a perfectly acceptable answer. I also think at some point, (maybe not now, but maybe so), she will be mature enough for you to tell her the truth (not that you should lie now, just don't share specifics) someday though..."Your bio parents were not able to take care of you b/c they were ill/addicted to drugs and it was so important to them that you be raised in a nice home with parents who could take care of you & love you that they brought you to us." would be fine.
At 9, I wouldn't bring these people back into her life, even if they are clean etc...maybe give her the chance to write them a letter that doesn't include any specifics for tracking her down, or get in contact with them yourself (or through an attorney) and tell them that she wants to know more and that you would like for them to write a letter to her without specifics that gives her a little more information about them. That may satisfy some of the curiosity and allow her to move on until she is more mature. I recall at some point, although I don't remember how old I was, that my parents let me read some kind of an info sheet on my parents that didn't have any identifying information(mine was a closed adoption) that gave me some basics about my parents (i.e. graduated hs, like psychology etc.) but not much more. That definitely was helpful in providing me with some info without really putting me in a position where I could have been hurt by direct contact. Maybe giving her the opportunity to write letters would help, I also recall that my parents used to send xmas pics/cards to the adoption agency which made me feel better because it assured me that if they wanted to find me there would be a trail so to speak...you could possibly allow her to do that through an intermediary.
All in all, I think it really depends on your daughter's emotional and mental maturity, as well as the intent/situation of the parents. So, really, there is no "right age." I imagine that there is the possibility that they could all meet, talk for a while & never want anything do w/ eachother again...but the possible consequences if that is not the case and someone wants more could be very harmful/dangerous to her well-being, particularly if they're still involved in illicit activity, and really warrant serious consideration of whether that would be the best thing even if you hold off for 5 or 6 more years. I definitely don't advocate lying, and I think you should be honest as is age-appropriate, but you are not obligated to make them and all their info available to her.