This is dependant upon the insurer and the policy itself. Many insurers differentiate between whether the child is still in full time education.
Often the maximum age for children on a family health insurance plan will be 21 years old if no longer in full time education or 24 years old if still in full time education. Other providers who do not differentiate may allow individuals to stay on a family health insurance to the age of 24 years regardless.
Again it is important to consider a range of insurers when considering a Family Health Insurance plan as the terms offered including maximum age limits for children should all be considered when choosing the most suitable cover.
You will need to either review your policy terms or call your health insurance provider to confirm the limits on your particular plan.