Answer : The route of drug administration is dependent on the patient's age because the drug response in body is found to be different for different age group of people.
A child may feel difficult to swallow big pills as they would not like the taste of the drug or the size of pill may get stuck in the esophagus tube. So, alternatives can be adopted for children's like injections, inhalations, etc.
For elderly people the digestive system acts slow; they must be given certain drugs which can directly reach the affected area and start its action.
Same way, Pregnant women can be at risk of harming the foetus if there is some overdose or a harmful drug is taken. Intravenous route can be beneficial for these group.