My oldest is 22. He's doesn't have any sort of cloudiness or cataracts some older snakes can get, but he is a bit on that leaner side with a bit of 'sag' on his lower belly scales.
He's definitely still fairly active when let out and will try to climb around or find a cozy place to hunker down in.
One of my other boys is a rescue that had some severe burns and scar tissue is definitely keeping him handicapped. He prefers to stay mostly immobile when moved out of his habitat, but he will still 'limp' around his habitat at times when he's feeling up to it. Definitely not a climber.
I agree with Bogertophis that it could be a combination of factors and not just 'age'. Most people when they seem my oldest boy think he's just a lanky female. The only other sign he has that might betray his age is just the spacing between his scales may be a bit more noticeable at times?