//Seems reasonable to me if they want to avoid getting Covid for whatever reason.//
Only if they are going to continue isolating for the rest of their lives, sqad. With Covid now endemic it is inevitable that everybody will be exposed to the virus, probably multiple times, unless they isolate from every other human being for the rest of their lives.
// My neighbour still wears a mask whenever he goes outside. Don't think he has any health issues.//
If the mask is of the usual type – poor quality, ill-fitting and worn, handled and treated incorrectly, it will do neither him nor anybody else any good. If he wears one of reasonable quality, wears it correctly, stores it properly and changes it frequently, it may provide a very small measure of protection to those around him. If he really wants to get any benefit from wearing a mask he must wear an FFP3 or FFP2 variety and wear, store and treat it correctly. This will provide him with 98-99% protection. And he must wear it continually for the rest of his life because, as above, exposure to the virus is inevitable.
Isolation is effective for so long as you practice it but it must be complete isolation. If anybody your family members mix with does not do likewise (like, for example, if they go to the shops) the rest might as well not bother.