Utility is an economic term for a person's overall well-being and satisfaction. Consumers play a role in the economy by maximizing utility, which means allocating their scarce resources, such as limited time or money, most efficiently. Utility maximization is different for every person.
You can start using it right away but it was suggested to us by our midwives to hold off until 6 weeks as pacifier use can mess with their breastfeeding latch (it's a different kind of sucking).
After the first few months, babies begin to gauge where things are in space, and they can plan an action, such as grabbing a pacifier. By simply dropping something and picking it up, your baby's learning that he can manipulate things with his hands and he's learning more details about how his toys work.