DS is almost 20 pounds at 6.5 months. I am looking to get a new carrier where we can wear him on our back. Right now we have the Bjorn and it only goes up to 25 lbs and doesn't do back wearing. When we registered for it we never thought we would have a huge child.
TIA!
Re: Carrier for baby over 20 lbs
If you like the concept of the Bjorn, look at a soft structured carrier like the Ergo or Beco.
I have an Ergo Sport and like it a lot.
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Yeah, my huge kid (21 lb at 6 months) is quite comfortable (as am I) in our Ergo Sport and our Beco Gemini (you really only need one...how I came to have and love both is a long story)
. I do slightly prefer the Beco for ease of getting on and off 
GL!
I used a SSC (Pikkolo) from about 6 months to 14 or 15 months. These days, with DD being more like 28lbs (22mo), I pretty much exclusively use a woven wrap. I wish I had gotten a woven sooner.
Ditto the ergo sport. We loved the bjorn for less than 15lbs, but once LO was 15+lbs, we upgraded to the ergo sport and it was a great decision.
My DD is only 20lbs at 13.5 months old and she is still comfy in the ergo. I love the back carry as well.
Ditto a Boba. I used woven wraps when DS was smaller, but they aren't as comfortable for me anymore. The Boba is great, we still use it pretty frequently, and DS is nearing 2. They are very similar to an Ergo, but the body is bigger.