Joe Giller wrote:

Hello All,
I have about 1.25 acres of ~35 year old Cab Sauvignon vines in the
Sonoma Valley that I took over tending and this is my first harvest.
Been checking the Brix roughly each week for the last 3 weeks and it
seems to be hovering around 19 Brix. I want to harvest this before
November if possible (when the rain might start coming) and was
wondering if perhaps grapes reach a certain Brix and stop (because they
are old or whatever). I don't mind waiting but just wondering if the
Sugar Fairy will come soon

There is more to "maturity" than brix.
Look at the stems and seeds - are they brown or green?
How do they grapes taste?
Do you have a pH meter?
What is the pH?