Pretty much anything, but the two most likely reasons could be blackleg and scouring. You better get your vet out to do a necropsy on the calf, as if it is blackleg, you better get those calves vaccinated for blackleg ASAP.
Disease or a blockage in the gut will cause a calf to stop eating. This is something you need to take up with your local large animal veterinarian immediately if you wish to have a hope of saving the calf.
That all depends on what breed that beef calf is. A 1 month old beef calf can weigh anywhere from 80 lbs to over 200 lbs or more.
This all depends on the breed. Three-month-old calves of different breeding will weigh more or less than the other. For instance, a Charolais calf will weigh more than a Jersey calf, or a miniature Angus calf will weigh less than an Angus calf (one that is normal in size and proportion).
6 months...
500
Around 300 lbs.
That depends on the breed. Since this is in the Jersey cattle category, the weight of a three-month old Jersey calf would be around 150 to 200 lbs at the most.
Depends on the breed and body condition of the calf. A 600 lb Jersey calf can be around 7 to 8 months old, whereas a 600 lb miniature calf may not be a calf at all, but a 15 month old heifer or bull. A soggy Angus calf at 600 lbs may be at around 5 to 6 months of age.
The most accurate way to tell the age of your calf is by looking at his front teeth. A newborn calf will have no teeth; a week old calf will only have maybe one or two teeth that have popped up already; a 1 month old calf will have all 8 lower incisors already.
Usually around 200 lbs.
Because the calf is very old. (So it doesn't die)
Depends on the breed and sex of the calf. Most calves average around 700 to 800 lbs at this age, plus or minus.