It’s a little weird that everyone else is the old age at which they died, but eh.
If we go strictly by in-universe logic, then Anakin’s Force spirit should appear as either Hayden or Sebastian Shaw in his mid-forties, since Anakin died at the age of 45.
For me both Shaw and Hayden are equally valid. My personal best choice, much like Burbin’s, is an aged up Hayden like the one we currently have in Ascendant. Hopefully a similar Anakin could be digitally put together to replace the admittely bad Hayden shots in the ROTJ special edition.
But anyways, if even Anakin’s accent changed that much then making himself look as old as Obi-Wan should be no issue for his spirit haha 😄