Langdon, not sure if you are asking how long after kidding the goat will continue to milk for, or how many years she will be able to be mlked for.
Some goats will run through and milk into the following year without kiddng again - but obviously less milk will be produced.
As to the age issue, many years ago we had a Toggenberg - nothing special to look at, but that goat gave us plenty of milk, and twins every year, and even triplets. She must have been at least 7 when she gave eleven and three quarter pints of milk.
Just wondering how you are going to manage all the goats on your land? Did you not get Mille and Molly in kid? You will obviously need to keep the male and female kids apart, as even if the female is not mature enough to breed, the male will not agree, and will be attempting to serve her - and also your other females.
Have you thought what you will do when the females are in season, because the young billy could and probably will try to get to the ladies. If you are not around he could cause some injury in his pursuit!!
I would definitely worm the newbie, and as a precaution I do keep them apart for a few days from the rest of my goats if at all possible - not that I think they are carrying any deadly illness, but its just a precaution.
Not being negative to your plans, just putting forward my thoughts and ideas. Best to think things through now and make plans on how to cope