In 1977, when Anakin and Vader were separate characters, I believe Anakin and Ben were peers of roughly the same age while Darth was much younger. When Lucas fused the two, it seems to me he initially leaned towards keeping the character around the same age as Ben. If Lucas hadn’t had a falling out with David Prowse, we maybe would’ve gotten Prowse in the unmasking scene, but though he was in his late forties during the filming of ROTJ, he might’ve been aged up with makeup. But when he started making TPM, he wanted Anakin as a young kid — a 12/13-year-old at first, then aged down to 9 to make his separation from Shmi more emotionally impactful.