I was looking around Youtube and came across a movie with a 21 year old adult baby!
Here is a summary: A social worker who recently lost her husband investigates the strange Wadsworth family. The Wadsworths might not seem too unusual to hear about them at first - consisting of the mother, two grown daughters and the diaper-clad, bottle-sucking baby. The problem is, the baby is twenty-one years old.
I have not watched all of it yet but I will and I will tell you what I thought of it.