Hi Susan: The current thinking on seated exercise is "don't do it unless you absolutely have to" no matter your age or bone health. The seated posture as compared to standing or lying down has three major disadvantages.
Place unsafe pressures on the vertebrae that can be avoided by performing similar exercises standing or lying down
Do not stimulate balance which is vital to falls and fracture prevention
Do not properly prepare the body for Activities of Daily Living (ADL) such as carrying groceries, getting in and out of a car and putting away dishes in the kitchen
The best exercise program is one that stabilizes and decompresses the spine and strengthens the whole body while improving balance and functional movement.