Looted this from cyberspace:
There is a myth floating around that adult artemia are not as good for your fish as newly hatched. There is a tiny bit of truth to this, but it depends on what you are feeding. So whats in it for your fish: Newly hatched artemia are high in fats, about 23% of dry weight. By mid juvenile stage, the fat levels have decreased to about 16 %, and by the time they are pre-adults the fat levels have decreased to about 7%. But, at the same time, the protein content has risen to replace the fat, from about 45% in a newly hatched artemia to about 63% in an adult. Based on this, you should determine what is best for your tank, young fish larvae require a high fat intake for growth and health, while older juveniles and adults need protein for health and reproduction. Also, nauplii are known to be deficient in several essential amino acids, while the adult artemia are rich in all essential amino acids. Adult artemia therefore supply more biomass than nauplii and are more nutritionally complete.