Generally speaking there is no upper, or lower, age range for playing Tai Chi.
Currently the Yuang Chi School of Tai Chi has a policy of only admitting those under 16 if they accompanied by a parent or guardian.
If you are older and you have some physical limitations on your mobility, this will not, generally, prevent you from taking part in a Tai Chi class. However, you need to be aware that the majority of each 2 hour class is done standing and if you have difficulty staying on your feet for this period of time you should contact someone at Yuang Chi School of Tai Chi before enrolling.