Okay hear me out
>people think old man Zangetsu looking younger during Bankai was a normal zanpakuto spirit, but that Zangetsu is a special case (for starters, he's not even a zanpakuto spirit)
>quincy!Zangetsu and shinigami!Zangetsu used to be in a power struggle over who gets to be Ichigo's main energy source, for example the third time Ichigo visited his inner world quincy!Zangetsu wasn't there because shinigami!Zangetsu had gained way more power and influence over Ichigo
>in other words, every time Ichigo meet old man Zangetsu he was always struggling to handle Ichigo's shinigami power, he was never at 100%

>old manga Zangetsu is a manifestation of Yhwach. And Yhwach's age is proportional to his power
The reason quincy!Zangetsu looked like a bishonen during Bankai isn't that zanpakuto spirits change during Bankai. It's that during Bankai shinigami!Zangetsu gets stronger, making quincy!Zangetsu weaker and thus, younger. Now both sides are accepted by Ichigo and no longer in opposition quincy!Zangetsu will never look like his bishonen self (when he was heavily nerfed) or like his "old man Zangetsu" self (when he was normally nerfed), instead he should look even older now he can be at 100%.
This is what Ichigo sees every time he checks his inner world and looks at old man Zangetsu.