You can do a little Taichi / Qigong or a lot, anytime you like, as long as your body says it's OK.
The video I posted is by Don Fiore, he has a bunch of great videos in his
Youtube Channel. He specifically tailors his Taichi forms for the senior audience. That is my target group as well(of course) for my practice sessions. I just started visiting the senior center in a neighboring community, folks there are late 70s and above, there is about 8 people that join in with a lady there that's leading a fitness session. She has invited me to add some Taichi into her class. I'll be reviewing my material to make sure it's compatible with her participants.