I noticed medication dosage for children is based on weight/age. what if you're a petite adult under 90lbs? would my body require less dosage for the same effect?

Not necessarily: Some medication are dosed by weight but not others. look at the bottle and follow the instructions given

Prudent: It would be prudent to adjust the dose to suit your weight, especially for drugs with narrow margin of safety. It would be prudent to discuss it with your doctor and pharmacist for specific drugs. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.