We have a Kelty frame carrier which I *think* is listed to 45 pounds for the child... which my son is just at... but we haven't had him in it since before he was 3 (pretty sure). The thing is that just like with car seats you need to take HEIGHT into consideration as well - at a certain point a child may be light enough to carrier in the frame carrier but might also be absurdly tall. Also carrying a child in a frame carrier can't *exactly* be pinned down to a matter of weight... if you are backpacking you may find 40 pounds of gear in an internal frame pack is "not all that bad" but those 40 pounds of gear tend to ride pretty quietly... as opposed to a 40 pound kid who moves around, yells, grabs for things, and tries to stand up.
I did like my Kelty when my son was small - but I can't really see using any frame carrier as long as some of them say you can. But it all depends on how active your child is and how tall.
My son is 4 and a few months and he's ~43 pounds and he's TALL. I don't wear him but I do still carry him... at 7-8 months pregnant with twins which tends to result in a lot of horrified looks from the general public LOL. I don't carry him a LOT but I do still carry him... and I am looking forward to my mai tei carriers coming so I can try him out on my back just for one last whirl. I don't expect to really use them with him but I feel like I missed out on wearing him as much as I could have. This is just my gut impression but I think that a mai tai or SSC where the child is getting more of a piggy back ride than in a frame attached to your back is liable to work out better for a bigger toddler/preschooler - who knows. But kids that age do still like to be carried... but can't be as squirmy when they're carried nice and close vs. in a frame where they can try to stand up and stuff.
So I am agreeing with the people who are saying carry/wear them as long as you want to.