My grandfather is 90 years old, and he was badly injured, how much compensation could he be entitled to given his advanced age?

No matter what ones age, we are all entitled to be free of injury or abuse.  The mere fact that one is elderly is no excuse for incompetent or abusive care.  Often those most ill-treated are individuals who are subject to dementia or Alzheimers and they more than any one else need protection because they cannot protect themselves.  Elderly people are entitled to the same compensation as anyone else.