I think this is a very delicate situation. Whether or not the adoptive parents help the child search depends on what the child wants, how mature the child is, and what the parents know.
For instance, my brothers' biological parents were drunk drug addicts who were on and off the streets who lost all 7 of their children. Some of the children they gave up at birth... some were taken away from them. They never managed to keep a child more than 9 months. My brothers are both mentally retarded from prenatal exposure to... well, no one really knows what all they were exposed to. The older one was also malnurished after spending nerely the first six months of his life in the custody of his biological mother before he was removed. The younger of the boys was born in a public toilet... she did pull him out and leave him at a hospital, so I suppose she wasn't the worst possible mother.
The younger of my brothers has alwasy wanted to meet his biological parents... who knows if they're even still alive. I know I would never help him in this search! He is not emotionally mature enough to cope with the possible consequences. He doesn't have the cognitive capabilities to realize what could happen. What if he tried to run off with her?
In fact, I think 18 is probably too young for most adoptees to be ready to meet their biological parents if they are at all conflicted about the idea. This isn't a universal rule of course, but I know how I felt at 18. It wasn't even MY biological mother I was searching for (yes, sounds crazy, but I did try to track down my brothers bio-mom). The thing was, what I wanted to find was a way to get revenge on her. I'm not sure what I would have done if I'd been able to find her... something dumb I assume. I had so much hate and anger in me directed at her. It is painful to watch someone grow up handicapped and hurting and knowing that it wasn't some random accident, but something that had been DONE to them.
Of course, every adoption situation is different. Open adoptions are usually the best way to get well adjusted children. In situations where that is not possible, however, knowing when it is a good idea to help your child search has to be a hard thing. Curiosity is fine, and should be satiated. However, I know if I ever adopt, I would be very very scared to help my child search for his/her biological parent if I felt like they were still consumed by fear or anger. 18 is still developmentally fairly young. Humans are still evolving and most studies say that adolecence brain patterns are lasting longer and longer in people. You want to make sure your child is truly ready for all the possibilities before you expose them to something that could be so emotionally jarring.