Why route of drug administration depends on age?

The oralroute of administration is the most widely used route and most acceptable for the patient. The oral route should always be the first considered route and alternatives to this route only sought if specific problems prevent this route being utilised. These include:

emesis because of irritation of the gastrointestinal mucosa by the drug; destruction by digestive enzymes or acidic gastric fluid; irregularities in absorption in the presence of food or other drugs; drugs may be metabolised by enzymes or bacteria in the gastrointestinal tract before systemic absorption can occur; absorption of opioids may be reduced by the delay in gastric emptying that follows surgery. This has the dual disadvantage of non-absorption initially, followed by the possibility of a large dose being absorbed when gastrointestinal function resumes.

hope i could help