My two cents....

First, and foremost, since the FICO algorithm is a proprietary trade secret, no one can put numbers on any one scoring criteria.

That also applies to category weightings.  While the standard pie chart shows length of credit at 15% of total scoring, all indications are that the category percentage itself is not always 15%, but varies by the consumer's scoring "bucket."  A consumer, for example, with perfect payment history may have increased emphasis placed on other categories, such as length of credit.

Throw on top of that the fact that all of the length of credit history is not based only on AAoA.  Oldest TL also counts.  I additionally believe that new accounts under 6-12 months or so are also part of the length of credit category.  What is included and the relative weights of each are simply speculative.

It would appear that reaching a high level in any one scoring criteria would have some kind of cutoff benefit, but it most likely slides around.  I think it is safe to assume the extreme... that a 50 year credit history wont give one an 850 FICO score.