If Donald Trump makes a claim it is notable. If my grandfather does, it is not. Unless my grandfather is Donald Trump (he was not).
How notable does an individual need to be before we accept that person making a claim by itself to be sufficient to consider the claim to be notable?
If we're going to assume that what they say is noteworthy anyway (may not always be the case, see the discussion Oddthinking linked), then I would say that otherwise the determining factor is how visible the claim is, because that is a good proxy for whether it is widely believed. Right off the bat, a Trump speech might start with around 47 million US viewers, and over time the viewership grows as people around the world watch. I assume your grandfather does not have such an audience.
Similarly, if J.K. Rowling posts something on Twitter, she will have a wide audience.
If the claim you are intending to evaluate has an audience of less than 1000 people, then I think it is safe to say it is not notable. Why 1000? The smallest country (aside from the Vatican City) has a population just over 1000. I think a claim that might be believed by a whole country (even if that country is Tokelau, a dependency of New Zealand) is worthy of consideration, thus notable.
I think I reject your premise. Not all claims from Donald Trump are notable.
This is discussed here: Should off-the-cuff claims by Donald Trump be considered notable without evidence that people actually believe them?
Rather than discuss how famous a celebrity is, let's remember why the heuristic is in place. The question to ask is "Is the claim widely believed?"
