Translation:The older he gets, the less he understands.
"jo... [jo/desto/dess]" is used for comparison in the same way as "the... the" is in English:
"Jo større, jo bedre."
"The bigger, the better."
That was illuminating, thank you! Not being a native English speaker I never thought at that "the" as different from a normal "the", but now that you point it out it does have a very specific function. And now, forgive me for my ignorance and for taking advantage of your kindness, but what are desto/dess? neutral/plural?
I'm glad it made sense! Sometimes it's easier to understand one language when you put it up against another. :)
"Desto" and "dess" are just alternatives to "jo" (in the second part of the comparison), so you don't need to worry about them agreeing with anything else. It's a stylistic/dialectical choice.
What an interesting construction! It reminds me of english "the ... the" indeed, though I'm not a native speaker as well. However is it always allowed to violate V2 rule in the first part of the sentences with this type of comparison? Correct me if I'm wrong, but "jo endre han" is not a complex collocation which forms a subject here, isn't it?