In many situations an adoption proceeding can be a rather lengthy process. Usually the parties looking to adopt have to undergo a rather intensive background investigation that includes everything from financial fitness to individual health matters.
However, in situations where the person seeking to be adopted is over 18, the process becomes much more simplier. Under current Indiana law, the person seeking to adopt an adult has to file a petition (joint petition if he or she is married) and the person seeking to be adopted has to consent to the adoption.
Now the court does have the power to order a background investigation (including home studies, etc) on the person seeking the adoption, but this is completely within the Court's discretion and may be bypassed by the Court.
Clearly, the reason for the simplified process is the fact that the person being adopted is at the age of majority for most purposes and can decide to enter into this on his or her own accord. But, regardless, the process is much simplier and can be handled more quickly than adoptions involving a person under the age of 18.