Your doctor can let you know what illnesses you need to be screened for and when, but here is a quick rundown. (Of course, this list doesn't take into account your family history or current health.)
Heart Disease: 55 years old and regularly after that, or with the presence of any symptoms or risk factors
Colon Cancer: 50 years old, for men and women done every 10 years
Male Cancers: regularly between the ages of 40 and 75
Female Cancers: Breast cancer- every 2 years between the ages of 50 and 74, Cervical cancer- every year after age 21
Diabetes: age 45, but sooner with the presence of symptoms or risk factors
Osteoporosis: women age 60 and then every 5 years