Kittens start to eat some soft food from about 3 weeks of age and gradually transition to harder food when their back teeth start to come in. By 6 weeks, they should be confidently eating hard food such as kibble. A kitten that is able to eat solid food on its own is at a good age for socialization in a foster home. If the mom is friendly and enjoys petting, though, the family can be moved indoors immediately. If you aren’t able to observe if the kittens are eating solid food, use this guide to decide how old they are, based on physical appearance, activity and interactions with each other.