"Are you referring to doing any intensity work at all, or just to doing mainly intensity work? What is "intense" to you? Anaerobic Threshold work? Intervals?
No, Bob. I was referring to Brian's emphasis on intensity as the dominant theme in training. IMO, intensity as in intervals or AT sessions, has its place in training to reach a higher level of fitness. In running, intensity meant hill work and intervals, either on the track or on the road. For climbing, it meant AT work on an uphill trail, usually with a light pack or no pack at all, but we also would include sessions on a step mill if there wasn't time to get out or the weather was REALLY cruddy. BUT, this kind of training always took place against a background of endurance training and had a purpose, either to race, or to function well at altitude or on climbs where the ability to move fast when necessary provided an added measure of safety. Doing it without a purpose, IMO, was just risking injury for nothing.
"How do you view something like doing mainly low intensity aerobics, with one AT workout and one interval workout per week? Do you believe that is likely to lead to injury?"
That seems to me to be a very sane approach, as a rule of thumb, that would achieve pretty much what I was talking about above with minimal risk of injury. Caveat: The risk and results will vary from person to person, and with the volume and intensity of the workouts.
"how does your answer vary, if at all, with the age of the person doing the exercising?"
Excluding the genetically gifted, intensity should have an inverse correlation with age if one is to avoid injury.