As young as possible, for a couple of reasons. Babies grow quickly, and your little one will never be this small again. What’s more, newborns are usually very sleepy and docile for the first few weeks, making it easier to get a good print. If you wait much past three months, your baby is more likely to protest, pull back his hand, or try to grab and explore the medium. Still, if reflexes make it impossible to get a good impression, his hand won’t grow THAT much if you wait a few more weeks.