I weaned some of my angus x (with charolais, or hereford) that are on older cows off at 4 1/2-5 months of age this year. I got premium prices for them. They weighed around 450 lbs. I fence line weaned them and the mommas did ALOT more bawling than the babies. With the dry year, it was a few more mouths not to be eating the pasture. The steers I kept are still on mommas, I probably won't wean them off till they are 8 months old.
I take my chances on sale barn calves sometimes if the pair is being split for the cow to be a weigh cow. I have gotten calves in the 200-300 lbs. range and although they may not be as growthy as their still nursing counterparts, they do pretty good with good feed. I don't see any reason you'd want to wean that early unless you are desperate to get the cow on the truck, but it does work out OK.