How long can you baby wear for?

As the title suggests really, is there an upper weight limit? I'm not sure my 18 month old would enjoy being worn anyway, he is pretty wriggly but I just wondered if you can buy wraps for heavier, taller children. William is more the size of a 2.5 year old!

Charlotte- I still wear the girls at 35 lbs- I use a sling- currently I am using a seven sling, which they recommend up to 35 lbs, but I have a friend that still wears her 40lb baby in one just fine- I am looking into woven wraps for longer term.