Apparently after 1 year of breastfeeding, the mom's body stops producing "cream" so to say... the milk drops in vitamin content, and so after a year or so breastfeeding becomes purely comfort to a child who is well able to get all their nourishment through solid food. Which, is the point, to exercize those teeth and digestive system, and to get a variety of vitamins and minerals through various food sources. Also, breastmilk is sweet and can rot a child's developing teeth if it continues throughout the day. A woman i know insisted on nursing her (at the time) 3 year old, and his teeth were concave at the ends, rotting away. She was in serious denial that nursing was causing this problem, and in her case it was apparent it was her inability to let her child grow up and leave her lap that kept her in an un-necessary clingy cycle.
Breastfeeding past the natural time frame of approx. 1 year also can create psychological problems for the child. The child can miss out on developmental opportunities to develop important independence from mom. Nursing children can be more clingy rather than outgoing, and if the child continues to nurse all the way until age 5-- that's school age! Guaranteed no other child in his class still nurses, and that might cause some embarrasment.