Final answer:
The combining form that means old age is 'presby/o', which comes from the Greek word for 'old' and is used in medical terms related to aging, such as presbyopia.
Step-by-step explanation:
The combining form that means old age is presby/o. This prefix is derived from the Greek word presbys, meaning 'old'. Combining forms like presby/o are commonly used in medical terminology to describe conditions related to aging. For example, presbyopia is a condition associated with aging that results in the inability to focus on close objects.