This points to a pretty obvious error in thinking.
I didn't say only old white people vote for Trump. We're a country of 330 million. People of every racial, ethnic, and age variety vote for every candidate. Ben Carson votes for Trump. That doesn't change that 90% of black people vote against him. Caitlyn Jenner votes for Trump. That doesn't change the fact that 80% of the LGBTQ community votes against him. If the vote were left to those groups, he would lose in a landslide.
But it's absolutely factual and undeniable that Trump's core support - the group with which he polls best and from which he most disproportionately pulls his support - is old white people. If the vote were left to them, he'd win in a landslide.
There being a middle-aged Asian somewhere stupid enough to vote Trump doesn't change the fact that most don't.