Those who are mentally incapable of accepting Jesus (ie, young children and the mentally handicapped) are innocent and still belong to Him and thus are saved. What if someone did not accept Jesus and was rendered mentally incapable of accepting Him after an accident? Or, similarly, what if an elderly person reached the point of dementia where he or she was mentally incapable of accepting Him?
None of us knows how much time we have. We could die, or suffer an accident or disease as you’ve described. If we’ve reached the age of maturity where we’re held accountable for our behavior, every day we wait before accepting the Lord could be our last one. If we don’t take advantage of the opportunity offered us when we can, we’ll have no excuse. But remember, God is just and knows the end from the beginning. Therefore He has to see that everyone gets at least one bona fide offer of pardon so an informed decision can be made. We all receive several such offers, but it’s best to take the first one we get.