Final answer:
The 23 gauge needle is most commonly used for infants and the elderly as it balances the need for medication flow with minimal discomfort and tissue damage.
Step-by-step explanation:
The needle size most commonly used in infants and the elderly is 23 gauge. This size is chosen because it is large enough to allow the flow of medication or fluid but small enough to minimize discomfort and tissue damage, which is especially important in the sensitive veins of infants and the elderly. There are various gauges of needles used for different purposes; a 16 gauge needle is considerably larger and more often used in situations where rapid infusion is necessary, while a 35 gauge needle is extremely small and not commonly used in standard medical practices.