Term limits have nothing directly to do with declining membership. Indirectly it might, but only if people taking the seats of members retired by term limits do a better job to make AMA attractive to new members - a distinct possibility, but not a sure thing.
There seems to be an inversion of logic in the argument that membership is declining because the average age of members is increasing. It is sensible in a way if you consider that members are more likely to die off as they grow older, and without replacement, the rate of decline should be expected to increase.
A more rational nexus between average age of members and declining rolls is pretty obvious: If new members aren't joining at or near the average age of current members at the time they joined, or fewer members are joining than leaving (a given, as membership is declining), then the average age of members must increase, as has been observed.
Young people are not finding AMA attractive and hence do not join, and that is why membership is both declining and getting older on average. Doesn't mean young people aren't interested in model aviation (PFs amply demonstrate otherwise), just not AMA.
What's wrong with that picture?

Abel