Hi, it's difficult to age guinea pigs. If they're not fully grown, they are probably under a year or so old, though they can grow a bit after that. Fully grown piggies have thicker nails, and older piggies may have curled nails and can start to get a bit bonier. Other than that...it's hard to tell. When I got Bobby and Bailey I was told they were about 6 months, but both have one or two curled nails and their hip bones are a bit more prominent, so I reckon they're quite a lot older than that. My other boys are about 15/16 months now, and they just LOOK younger, and still have quite little nails. They are much more excitable and energetic too. Bobby and Bailey don't do as many zoomies and popcorns. I hope this helps a bit.