B cells are a specific type of white blood cell called a lymphocyte. Other types of lymphocytes include T cells and natural killer cells. B cells develop from stem cells in bone marrow. They remain in the bone marrow until they become mature.
Infantile behavior is something the masses are taught to exemplify since childhood. As adults they (mostly Betas) are encouraged to have sexual relations with multiple partners.
A small child is commonly referred to as a baby irrespective of his age though there are developmental stages in the first few years of the life of a human offspring. The smallest of babies are the newborns in the first few days of their lives. Infant is a word that is reserved for a kid before the age of 1 year. So, if you have a baby boy or girl who is a few months old, the doctor would refer to him or her as an infant. However, there are many grandmothers and grandfather who would take exception of this definition and call their grandchildren babies even when they are much older because of their extreme love for their grandkids.