YARN
Fleet Captain
Kirk still got fat and old! Case Debunked! LOL
He had to get older for the illusion to be semi-plausible. Captain Kirk always had a weight problem (recall the salad diet he was put on in Corbomite Maneuver). Getting heavier and older is all just part of the fantasy. These are issues he's worried about struggled with. He's vain. He never wanted to lose his looks or grow old. TWoK through UDC, again, amounts to Kirk working through these psychological issues.
And notice, even as he gets old and fat he still has his "mojo" at least until Episode V. I submit that the only way Kirk has his mojo in the "Search for God" is if his reality is already some sort of massive illusion.