The short answer is about 6 months old. As hiking carriers are typically carried on your back, children need to have good head control to sit in them unassisted. It's important to ensure that your child is well-supported while in the harness and isn't being put under any undue strain or pressure. To assist your child with their head control in their formative years, be sure to give them plenty of floor and tummy time! If you're using a front baby carrier, you can likely go hiking with your baby when their younger, but check with your doctor.