How many calories does a 51 year old man need to take in?

These questions all depend on your level of activity.

If you are an "average" person, you should be eating 1700 to 2500 calories a day with no more than 1/3 of those calories coming from fat. If you are really active (think daily runner) you could get away with more.

One pound of fat is 3500 calories to burn.

Please, before taking health advice from strangers on the internet, consult your doctor.