Thank you for asking and answering this. I had the same question this morning.
A Google search wasn't very helpful, most pages said simply "you" must be 65 or older and completely glossed over MFJ. The only page I was able to find that explicitly talked about age differences was somewhere on AARP (not exactly my go-to for solid information), and that said "To use the new form, filers, including both spouses, must be at least age 65" (bolding is mine). To be fair that may have been written while the form was still being developed.