I'm not mixing average with handicap. WGT, however, is mixing average with whatever they define it as.
A score shot over 2 (or 3) years ago should not count as part of current average. Sandbagging over that amount of time simply isn't going to happen unless the person is also a multi-accounter. In that case the current system won't stop them anyway. They already don't respect the rules and will simply sandbag a new account.
Age progression causing a decline in skills should be reflected in average. The way this game is, if the game survives for any length of time a person who shot 65s when they were 60 will be stuck with that when they are 75, suffering from macular degenerationand a decline in motor skills.