True, but Obi-Wan said that in ESB and what Lucas gave us in TPM didn't even align with that given the whole Qui-Gon making him promise to train him and the Jedi Council approving it. It really wasn't Obi-Wan's decision at all. If it wasn't Qui-Gon's dying wish he probably would have dropped off Ani at the nearest adoption agency lol. Plus, in ESB didn't Obi-Wan also point out that Luke wasn't much older than he was when Yoda first trained him? So Lucas really didn't have to begin Anakin's arc at such a young age either.
If the PT had started with a ROTS-aged Hayden I think that would have been better, as the age difference with Obi-Wan would have been less jarring, he and Portman's relationship wouldn't have been seen as creepy and it would have better aligned with the OT Obi-Wan/Vader dynamic.
Regarding the Shaw/Guinness age difference, yes Shaw was older but all you really saw of his Anakin was an extremely disfigured head. Given his injuries and scars one could buy his being either older or younger than Obi-Wan. As for his Jedi Ghost look, given what he went through becoming Vader it stands to reason he wouldn't age as well as Obi-Wan lol.