Why Alpert never APPEARS to age
This came to me when watching Widmore talk to John. He says he saw him 17 years ago and John hasn’t aged a day. It seems Widmore was describing the same thing that the audience has been seeing with Alpert since season three.
Alpert’s agelessness is simply due to him being disconnected in time. When we’ve assumed he’s the same age year to year, for him, it’s actually one day after another. Just like it was for John when he saw Widmore four days (17 years for Widmore) after their island encounter.