I think calling this a plot hole is overdoing it- in relative ages of Ewan to Alec, it's only a nine years discrepancy, which is well within the scope of drama and all the more understandable as Ewan had to play a ten years younger Obi-Wan also.
I think people over-estimate how old Obi-Wan was being portrayed as in ANH. Alec Guiness was only 62, and they made no attempt to make him look older.