flubenvet and chicks

Hello all!
I am new to this forum and I would be very grateful for any advice.
I ordered some flubenvet (in the form of ready-mixed pellets) before my chicks hatched but it didn't arrive until after they hatched. They are now 4 weeks and 3 weeks old.
The adult chickens including the mothers, have shown signs of worms so it's more than a systematic treatment, I'd really like to address it.

So now I have a kind of dilemma-
Is there a mimimim age to worm chickens? I can't find the answer anywhere.

And, I can't see a date on the packaging but I have heard that flubenvet goes 'off', anyone know how long I've got?

Is there any point at all in treating the rest of the flock, if I'm not treating the 2 mothers and 9 chicks which in a few weeks will be reintegrated?

Thank you so much from me and the chooks, I am very excited to be part of this forum and chatting with like-minded people

Thank you Annie, wow I have a new Grannie! 
That seems sensible, and then I can reintroduce the rest.
The chickens are completely free range in the day time, they can literally go wherever they like as we live in a very isolated place with no roads or neighbours. But this does mean that the worms are likely to survive in the ground as you say.
I will follow your advice and worm the main flock.
Do you think there is any way I can worm the mamas without the chicks getting a dose?
I was considering getting some of the more concentrated flubenvet powder and giving each of the mothers a raisin full of it each day or something..?
Has anyone tried something like this?
Thanks for your help, you won't believe how long I've spent trawling the internet for answers.

Thank you Annie, wow I have a new Grannie! 
That seems sensible, and then I can reintroduce the rest.
The chickens are completely free range in the day time, they can literally go wherever they like as we live in a very isolated place with no roads or neighbours. But this does mean that the worms are likely to survive in the ground as you say.
I will follow your advice and worm the main flock.
Do you think there is any way I can worm the mamas without the chicks getting a dose?
I was considering getting some of the more concentrated flubenvet powder and giving each of the mothers a raisin full of it each day or something..?
Has anyone tried something like this?
Thanks for your help, you won't believe how long I've spent trawling the internet for answers.

  Megkaya, I don't mind putting you on my list of adopted grandchildren!  I have 8 of my own, plus a great granddaughter of 7, plus loads of others, either virtual, or friends of my eldest granddaughters!   

How many garlic cloves do you put in the water New Shoot? One of my older girls will eat crushed garlic but the other doesn't like it. Haven't tried the 11 week old ones yet.

I have wormed the adults. NExt time I will buy the powder to put in a grape, instead of using the pre-medicated pellets, so I can let them continue being free-range! Also I've since heard of people adding dried garlic from equestrian stores to the feed as a preventative measure.
Thanks all!