he was like 20 in ep1
about 30 in ep2
kenobi will be in his mid 30's by ep3.
so, when ep4 rolls around, he will be in his mid 50's
and i disagree with kenobi looking "too old"
the stress of war and tragedy will age a person far beyond their years.
dont believe me? just look at historical photos of abraham lincoln from
before and after the civil war. in that relatively short amout of time, he
seems to have aged like 20+ years.
thats why ben's ancient look has never bugged me.
the man was under such stress.