Health problems

If you have a health problem, an injury or a history of physical neglect, you cannot reasonably ignore this when commencing tai chi.
It is important to start sensibly and safely.

Be honest.
If something feels uncomfortable, say so - serious health problems may well preclude you from learning tai chi.

Ignoring a knee or back injury and trying low stances or self defence is a sign of recklessness.
Allow for your age, your health, your fitness and your condition - and train patiently and carefully.