Hiya, thanks for the question!
First of all, the 40 pound maximum weight limit includes the total weight load, (the child, gear and pack).
Second, there is no recommended size or age. Your baby can fit in this pack as long as he or she can hold their head up and weighs 16 pounds.
The fit is determined by the weight of child. The child weight limit is a minimum of 16 pounds and maximum of 30 pounds.
If you need something bigger, the Kelty Pathfinder 3.0 Kid Carrier has a weight range of 16 to 40 pounds.
You can find that here: http://www.moosejaw.com/moosejaw/shop/product_Kelty-Pathfinder-3-0-Kid-Carrier_10189325_10208_10000001_-1_
Hope this helps!
Date published: 2016-09-14