Skip, Age probably is important primarily as a lifetime of disuse, misuse, wear and tear can rear an ugly head. Aside from some type of physical issue we're pretty adaptable to a range of crankarm lengths. A shorter crankarm is desireable if there is a joint issue with range of motion. Aside from that, a longer crankarm will require less rider effort for a given torque output because of it's mechanical advantage over a shorter crankarm.
Brad