Age is sure a factor but if that calf is 2 months younger why is his mother still in your herd.  The right question is which is the exception and which is the rule?  If the 600 lb  calves are the exception that is a bonus if they are the rule are you going to cater to the cow with the 400 lb calf, someone else is carrying her weight.  What about opportunity to net more lbs per acre.  Simple math would show that if you are running a herd average of 1300 lbs and you have 500 mother cows you should be able to run 540, 1200 cows.  Just to make it simple, we will say it takes 20 acres to run a 1300 lb cow and a 1300 lb cow weans a 650lb calf and a 1200 lb cow weans a 600 calf.  The 1300 lb herd will produce 32.5 lbs/acre while the 1200 lb herd produces 32.4lbs/acre.  Maybe the more moderate cow is not the answer.  It looks like to me, in order to increase production it doesn't matter the size of the cow if you have the same criteria but how many acres you can run her on.