I'm assuming you're talking about a sling or "pack" child carrier that you wear, not a carseat... Right? I had a sling for my daughter that you supposedly could wear on your back, but I just couldn't get it figured out - even with the instructions - she seemed too loosey goosey back there for my comfort. I wore her in front and on the side, sort of like carrying her on my hip, until she was about 6 or 7 months. At that point I put her on my back in an ergo pack instead - I still put her in the front when I wanted her to nap. Basically, they can go on the back as soon as they can hold their heads up and look around for long periods of time and are able to sit up. I hope this helps. :)