I've kinda been wondering about this line from ANH.
No other time in the films is there any indication that age weakens a persons ability to use the force. Yoda was 900 years old and he was pretty strong in the force. In EP2 Dooku was at least as old as Obi-Wan was in ANH and he was strong! The Emperor was older than Obi-Wan in the OT and he was certainly strong!
In fact, the indication is that Vader would have been weaker because he lost part of his body. Considering the fact that Obi-Wan defeats Anakin in EP3 it would seem to me that he should have been able to defeat him in ANH.
Yet Vader says "Your powers are weak old man."
He seems to be implying that Obi-Wan got weaker in the force with age!
It just dosen't add up.