I don't think there is a minimum age that you can trim a piggies nails, if they are long they need trimming. If this is their first time getting their nails cut, I'd try to make it a positive experience. At first, you don't have to trim all the nails, just introduce them to the clippers.
A lot of guinea pigs, including my piggy, hate nail trimming time. And it can be a real struggle to trim a piggies nails if they don't want it done. So hopefully if they are introduced the to clippers positively, they will learn it's not so bad to have their nails cut.