The average weight of a slaughtered cow is between 1,010 lbs and 1,321 lbs although A 1,321-lb. cow at time of slaughter is a pretty big cow, but that's not the cows mature weight.
Cows that are under the age of 36 months of age have the highest quality of beef so that's usually prime time to slaughter the cow before they get over 3 years old.
But older cows can also produce acceptable beef if they're fattened properly but calves are best slaughtered between 3 weeks to 16 weeks of age.