(A) is a very, very sneaky and tempting wrong answer —
The difference between it and the original argument / answer choice (D) is that –
While the original and the correct answer compare two different groups (areas with higher altitude vs those with lower; older trees vs younger),
Answer choice (A) describes different points of time for a single entity (the same person at an older age vs at a younger age) —
(A) would match the original argument better if it in some way compared older people to younger (“the older a person is, the wiser” or something like that — notice the change that happens in meaning from the subtle switching of “gets” for “is”).
Hope that helps clear things up and let me know if it doesn’t and I’ll be happy to discuss further — Mike