You can start practicing Qigong even if you aren’t in the best of shape or health and you will see improvement with consistent practice.
The health benefits of Tai Chi and Qigong are very similar but Qigong can be more suitable for older adults. The movements of Qigong are simple and repetitive and do not involve a complex series of long movements that require memorization. Qigong forms do have sequences of movement but then can be performed out of order and still have a profound effect.
One of my Qigong teachers has a wonderful story to validate this. He often will have people approach him at workshops who have been studying his methods at home. They will tell him how much they are benefiting from doing his work and will demonstrate what they are doing. Frequently the movements look nothing like what he has taught and yet clearly they are benefitting.
Point is, when you are aware of the basic principles of Qigong, body, breath, and mind and align with the forces of heaven and earth, you will experience changes in your health and well-being.