The age thing is a bit of a bugger, going by Sir Alec's age Obi-Wan should have been 43 in the ROTS time line.
Ewan's only 39 now (ROTS is already 5 years old, doesn't the time fly?).
I suppose you could put it down to premature aging because of two suns blazing away.
They should have cast Kenneth Branagh (even though Ewan was one of the few decent actors in the bunch).