How can you tell the age of your chihuahua?

It's no different that any other breed or mix of puppy. The easiest way to check is by looking at the teeth. If the puppy is only starting to get get teeth (and has blue eyes that clearly cannot focus), you have a 3-4week old puppy. If they are all sharp puppy teeth, then you have a dog under 4months. If most adult teeth are in and they are bright white and perfect, you most likely have a puppy 5-7months old. If there are well established adult teeth with any yellowing imperfections, you know you have a puppy at least 1 year old. Your vet can certainly help with this. In some cases, vets can check by looking at the dogs' eye.