Although many children with Downs share alot of the same behavior, physical, and personality traits, they can not be lumped together as a whole. This is just like the typical COPD, FTT, add your own dx here, are all alike. Children may or may not have defects in every body system (in any combination), have the physical downs look, or the palmer crease. OR to add more confusion any one of the different genotypes of downs (trisomy, mosaic, translocation).
The degree of disability will directly influence behaviors, as well as past experiences (just like with anyone). The degree of MR results in the child like sunshine personality, but also the agressiveness. Just think of any 2y/o you come up to with a needle...they are goining to fight esp. if they have had a shot recently. A child of 12 may be in the same comprehension level as a 2 y/o and not be able to understand the need for the shot other than that HURTS me --> it is BAD. A child that has experienced alot of pain from healthcare providers as a result of the many possible physiccal defects will react totally differently from the child that is in the hospital for the first time.
The scope of MR in downs is HUGE. One child may be profoundly MR/DD, with little independant function. Another may be so mild that they drive, attend college, and funcion almost at a normal level or have none of the common or major defects seen in downs.
The setting you pratice in may cause you to see more of a certain type of downs. For example in an ICU you may see the worst of the worst with severe and or multiple congenital defects, in early intervention you may see the whole specturm.
Overall downs is a very diverse and extensive dx to deal with. I like to think of it like snow. Every one has the mental picture of a snow flake and it is pretty close to the same image for every one, but really no two snowfalkes are identical.
just my two cents.