A baby will start to self-wean, drink less milk, upon introduction to solid food. With solid food in their diet, they will want to drink less and thus will nurse less.
Yes it can. They will usually do this between six and nine months of age. The baby is ready to stop drinking from the mothers breast and will just stop.
Some babies close to a year or after become impatient with breastfeeding. Be careful that it is not for teething or an illness, as these can make breastfeeding uncomfortable.
Yes, a baby can choose to wean at any time. If your child is under a year, though, he may be on a hunger strike due to illness or even teething. Keep trying to nurse, and hold your baby often. If your child continues to want to wean, then let him. It is easier than when you choose to wean.