This is a good read if you are interested in looking for info on seniors beginning to exercise.
As with all ages and fitness levels, it is worth noting that it's often more a case of individuals having restrictions than any one exercise being 'bad' for everyone.
We get to work often with seniors of all fitness levels, from post op and physical therapy graduates to more than a few that inspire and motivate everyone in the classes, including our instructors, to keep active and keep young in heart body and soul.
Everyone has their own entry level. If you have any concerns, about where that entry point may be or if there's a valid reason for avoiding certain exercises the best place to start is with a conversation with an experienced trainer who can offer advice, modifications and peace of mind as you commit to being as young as possible for as long as possible.