I don't think there is an 'age limit' on using a sling. Most sling manufacturers will say that you can safely use their slings until your child is 30 lbs. Most babies/toddlers will outgrow the desire to be 'slung' before they outgrow the ability for the sling to handle their weight. It depends on what you and your child are comfortable with. I'm still wearing my son (mostly on my back in a mei tai) but sometimes in a sling at 22 months of age with no signs of stopping.
As he is getting bigger and heavier I have been looking into a more 'manufactured' carrier such as a Ergo or toddler Patapum which are made to hold more weight.
Happy Babywearing!