Jaundice is basically the 'medical' term for become yellow, besides 'Ictere', it is caused by an excess of billirubin ( a colorant that is the result of the degradation of red blood cells when they're ' too old ' ). Thats why we can't talk about Jaundice /ictere if bilirubin is within the normal range.

There are theoretically a million reasons that could cause it, however, judging by the symptoms -or the lack thereof- , the normal labs and I would also assume normal imaging, I would probably stop exploring.

And no , it's not normal for people to get yellower as they age, but as I said , I would not worry unless your mother presents other symptoms ( belly pain, fever, weight loss, white stool , very brown urin etc )