Let me give you my quick summary of being in this situation on the adoptive parent end.
I believe it is my job as an adoptive parent to support reunion when my child is ready and willing and I have. I also believe that as a parent, while he wasn't ready, it was my job to communicate with his birth/first mother, passing on the info that he wasn't ready and ask that his wishes be respected and he be allowed to initiate contact (he was always fully supplied with her contact info) when he was ready, and to request that she not push, force or attempt any sort of contact ambush when he had expressly expressed that he did not wish it at this time.
As things went for us, he was ready and did want contact right when he turned 18 and we helped make it happen for him as soon as possible. If that had not been the case though, I don't know that my job as a parent would have changed from one day to the next just because his 18th birthday had passed. The growth and independence process is a little more fluid than that.
There is a point where the contact between the child and the birthparent does not involve the aparent at all and it's appropriate with an adult adoptee that that would be the case. But I'm hesitant to say that the magical 18th birthday makes it that precise moment. There are many successful reunions that have been initiated through positive contact with the aparents, even with adoptees in their 30's. There are also many cases where the aparents were not truthful with either their children or the birth/first parent and just tried to sabotage the possibility for contact.
I would hope that your child's parents would be the former and not the latter, but I think we're looking for clues you might be able to supply that might help us decifer the situation a bit more for you.
That being said, it is all guesswork in the end. All we can do is each to be honest, ethical, open and respectful and hope for the same in return.