How the route of drug administration chosen might be dependent on a person's age

In elderly people, the gastrointestinal route is not preferred because their gastrointestinal tract is not so motile, and gastric acid production is reduced. The pH in the stomach rises, which affects the absorption of drugs. Thus, this route is not preferred in older people. Intravenous delivery of drug may be done for faster activity and targeted delivery of the drug. 
Children can face problems in swallowing of the drug, therefore, inhalation and sublingual administration, as well as injections of the drug, can be given.
In the case of pregnant women, route of the drug is chosen so as to prevent baby from its effect, if the mother is being treated.  The dose of the drug is also very important.