DS was 26 pounds at 18 months, he was 7 pounds 14 ounces at birth
DD is only 12 months, but she is around 18 pounds now, if she is more then 20 pounds at 18 months I'd be shocked, she was 7 pounds 5 ounces at birth
That being said, I can still carry DS on my back in my carrier, at 3 and 32ish pounds with no trouble and for a couple of hours if needed. You just need a good carrier to do it with. I have a Calyx that worked wonderfully for him at 18 months along with a Beco Butterfly. I currently use a WAHM brand for him, since it is made for older toddlers and preschoolers.