Several models of the heavier child-carriers are available although they are costly. Shop carefully, or inquire about rentals. Look for strong construction, plenty of adjustability to ensure your child won't outgrow it quickly, ample padding in the straps and hip belt, and storage for the many items you'll need to carry. Take time in the store to try it on with a real kid on board. Both of you need to be happy with the pack if you hope to enjoy your hikes.
Children develop at different rates, so it is risky to generalize.
But you can probably count on carrying your youngster on most hikes until the age of three. After that, you'll need the pack for trails longer than a couple of miles, and those that involve much elevation change. From five on, with judicious trip selection, your child should be able to make it most of the way on his or her own. Even if you are an experienced backpacker, carrying a child on your back will pose new challenges. Kids move, throwing you off balance at critical moments.