I came across this interesting note in the library at the University of Washington. Soman (1991) states that while maintaining a large group of tortoises together, "a small, three to four year old tortoise was found dead with its head eaten. On each of the following two days, one more tortoise was found dead. All three tortoises had lost only their heads. Shrews sometimes kill tortoises, and on the fourth day a careful watch was kept. It was observed that the old male attacked the smaller tortoises (all 3-5 years old). In all four cases only the head was eaten; the legs were retracted and uninjured."
I doubt that this sort of behavior will be found commonly in herpetocultural collections, but it is one thing to consider before keeping tortoises of widely differing ages together.
Soman, P.W. 1991. Cannibalism in the star tortoise Geochelone elegans. Journal of the Bombay Natural History Society 88(1): 118.