Mites How old do pigs have to be to be treated with Ivermectin?

I have two guinea pigs and they're both a month old. I'm 99% sure they have mites, or some form a parasite. They scratch every day like a dog. It looks very distressing so I want to treat them with Ivermectin. I'm not sure though, if a month is old enough. Also, I know that it's very important to give them the exact dosage so you don't overdose them. If I go to the vet, will they weigh the guinea pigs or something and give me the exact amount? Sorry for the questions but I'm a first time pig owner and not familiar with mites ..