For newborns and infants, there is really no difference between comfort nursing and “needed” nursing. If a new baby wants to nurse, it’s because he or she needs to nurse, whether or not it is for strictly nutritional reasons. (See my article “Comfort Nursing – Newborns and Infants” in related links at the end of this article.) But with older babies, after about 8 months or so according to many child development experts, wants and needs can begin to diverge.
Up until about 8 months, babies don't "manipulate" for wants, they simply communicate needs, which are indistinguishable. So the concept of nursing for comfort can begin to diverge a bit more from nutritional needs. That's not to say you should all of a sudden put them on a schedule and deny them all but rudimentary nutrition. For the most part, nursing on cue will still serve you well when it is possible.
After about 18 months or so, in my experience, babies can start to get a bit bossy and possessive about who decides where and when nursing should happen. This is the time to really start to think about breast manners, and how *you* are feeling about the nursing experience (see my articles on Breast Manners, Conflicting Feelings About Extended Breastfeeding and Nursing a Two-Year Old).
Just because a baby is older doesn't mean you cut out comfort nursing, but in my opinion, it is ok to ask them to wait, or explain that they just nursed recently and milk is not ready yet, but you are making more for later. At these times, I offer a drink of water as an alternative, or redirect attention to another activity.
But I don't know what I would have done at that age without nursing when my daughter conked her head on the table, was mistreated by big sister or had a fever. This is true comfort nursing, and nothing works better. She got not only that wonderful warm full tummy, but focused attention from mommy, which is just what she needed.
The first time after they weaned that each of my daughters was sick, and the first time they skinned a knee, were vivid reminders of how much both of us were able to rely on nursing to bring us together in these moments. We connected, of course, through kisses and comforting words and caresses, but I had so much appreciation for the way that nursing had provided us with so much more than calories, just how right and natural comforting through nursing is, and just how hard that instinctive comfort is to replace.
Comfort nursing is a great benefit of breastfeeding, and is always appropriate at younger ages. At older ages, non-nutritional comfort nursing is a negotiation between mother and child, and is certainly wonderful to the extent that is mutually enjoyed by mother and baby.