Because you need to take into account several factor, including the speed with which a drug may affect those of certain ages, the effectiveness of absorption or use, and the physiological differences between groups. If you are giving a dosage of a certain medication intravenously that would normally be balanced out in a normal adult if you inject it intravenously, you' d need to use a faster route for geriatrics like IV or IO (Intravenous or Intraosteo), shale you might need a slower route like ingestion for pediatrics.
