While the celebration may be delayed by a couple of years, or decades, I can't see a reason that the delayed celebration should not be permitted. There may have been extenuating circumstances that prevented the original celebration from occurring. I think it is only fair to give this man a chance to live out all his dreams before his passing.
Yes, Bar Mitzvah should be celebrated regardless of age. I believe God is not some kind of bureaucrat, who punishes anyone who isn't strictly following set rules. I believe it is more important that the person has an authentic desire and commitment toward God than following some rules. Look at the essence, not the form.
Normally I would agree that a Bar Mitzvah should only be for those who are just entering adulthood. However, in this case, the man's original celebration was canceled because of the onset of World War I. Surely an exception can be made in the case of somebody who has lived so long and done so much. Let him party!
So many sad things happen in life that it is never too late to celebrate a good thing. In life there is more sorrow than joy. This man should take advantage of this experience while he can. There is always time to find a reason to be happy. He has made it to 100.