No a suicide attempt at age 18 would not preclude someone from adopting years later. Although a background check is required to adopt, attempted suicide is not something that would appear in a criminal background check so it would be up to the individual to disclose the information. If he did disclose the attempt, he would only need to show that he sought treatment for his depression and that he is now doing well. My husband and I adopted our son through a private agency (not through foster care) and we had to provide a note from our doctor saying that we did not have any mental or physical health reasons that would make us unable to parent. As long as the potential adoptive parent is managing a health issue in a satisfactory way they are usually given the green light to adopt, as long as everything else in the home study goes well.