The sibling comment says "fire them". That sounds glib, but it's the correct solution here.
From what you've described, you have a coworker who is not open to learning and considering alternative solutions. They are not able to defend their approach, and are instead dismissive (and using an ageist joke to do it). This is toxic to a collaborative work environment.
I give some leeway to assholes who can justify their reasoning. Assholes who just want their way because it's their way aren't worth it and won't make your product better.
From what you've described, you have a coworker who is not open to learning and considering alternative solutions. They are not able to defend their approach, and are instead dismissive (and using an ageist joke to do it). This is toxic to a collaborative work environment.
I give some leeway to assholes who can justify their reasoning. Assholes who just want their way because it's their way aren't worth it and won't make your product better.