Is it really a criticism though? There’s nothing of merit to it. I could see criticizing the pick based on Walz’s views (though I’d probably argue against that too; Walz has a pretty stellar record).
He’s basically the same age as Harris, so the “old” portion of the criticism is right out the window. What we’re left with is that he’s a white man, which is always what the VP pick was going to be. The unfortunate truth in this country is that a lot of voters are not ready to support a presidential ticket with two women or people of color.