Hey, I have 2 female guinea pigs that are pregnant, we found out that they were pregnant after we got them. One of them is around 9-10 months, and it's her first time so were abit worried about that one. The other is about 3, she has given birth to 2 litters before but I've read some websites and they say you shouldn't breed them if they're over 2-3 years of age, and you should breed them for the first time before they're 7months. We're also concered that they haven't had them yet, we've had them for 9weeks now.

I am far from a guinea expert but I have done some searching for you come up with some sites you will hopefully find helpful.
It seems guineas can go up to 70 days some even longer so they should not be far from giving birth if they were already pregnant when you purchased them.

Pregnant Guinea Pigs

Home-Breeding Guinea Pigs

Another baby fact that may surprise you is that newborn Guinea Pigs are ready and able to eat lettuce(and other things) right after birth, and are HUGE when born.

It's okay guinea pigs are able to breed at 6 weeks but it is recommended for them to breed beyond 6 months, no need to worry if you think the younger guinea pig will be unable to care for her young because it's a natural instinct for them, if it is her first litter you may need to step in from time to time to make sure that they are being fed.

Guinea pigs should not be bred for the first time after they are sixmonths old asthere hips will set and they will (probably) not be able to give birth properly which can be fatal to the sow and pups. Also please don't feed lettuce as guinea pigs are allergic and ice-burg lettuce could kill them.