This is really based on personal preference, but usually around two years old. This is because as your child starts getting to be more mobile and walking, a onesie may start to feel uncomfortable. Also, the main purpose is to give your baby something cute and comfy to wear that gives the parent easy access to diaper changes. When your baby gets to be around two years old, they’ll likely be in pull-ups or potty training. At this point, the onesie isn’t as convenient to use. However, there’s nothing wrong with keeping your child in a onesie longer if they still fit and seem to be comfortable for your child.