Which of the following is not positively influenced by physical activity? a. cardiovascular disease b. obesity c. osteoporosis d. allergies

D. Allergies. 

Cardiovascular disease is disease of the heart -- the more blood the heart pumps, the stronger it gets. Obesity is a disease of the flesh, and the more calories lost, the less flesh there is. Osteoporosis is a form of arthritis which afflicts the bones, and special exercises can help the user remain more limber.

Allergies are just allergies, man. You can't outrun nature. It's everywhere. 
Hope this helps

The answer would be age 18.