I tried. When my son was three, I had him share a drink with my sisters son who had chicken pox.....and he didn't get it. Then, a few months later, a boy in my son's preschool had them and he was in the office waiting for his mom to take him out of the school so I had my son hug him.......and he still didn't get them. Last year, my husbands friend had them and I while he was still contagious, I had my son drink out of his drink and he was sitting next to him.......and he still didn't get them. He has a very strong immune system because the way I feed him and all the antioxidents he gets everyday and can't even catch a cold. But every chance I get, I will expose him. I really don't know what is a good age is to expose a child, but I wouldn't expose a child (in my personal oppionin) untill the child is at least two.