fishmanlee said:

Is there much a difference between 860 and 900 years old though?

Enough for Yoda to look very different in the PT than the OT.

Regarding whether Vader should be Hayden of Sebastian it comes down to whether he should look old or young, not whether the audience can tell that the unVadered actor is meant to be the guy from the suit, because if you watch 6 films then both actors are recognisable, and if you only watch the OT then only Shaw is recognisable. However, if Anakin was meant to be shown as a young version of himself why did Lucas not cast a young actor at the time. He had already cast Shaw specifically for the force ghost. This isn't something where the excuse 'the technology wasn't available' works so it seems that instead it was done purely as an afterthought to try to drag aspects of the PT into the OT. If he had included Hayden but aged him to look as old as Shaw did then fine, at least that still follows the original plan, but to make him young just because that how he looked last time he was good and we have to be able to make that link ourselves, well that seems to underestimate the audience a little bit in my opinion.