Age in Years vs. Total Days Lived: When Each One Is What You Actually Need
Age in years, months, and days is the right format for identity, eligibility, and everyday context — it's how people naturally describe age. Total days lived is the right format for precise milestones (a 10,000-day birthday), scientific or medical calculations, or comparing durations that don't align neatly with calendar years.
"How old are you" and "how many days have you been alive" are the same underlying fact expressed two different ways, and picking the right one depends entirely on what the number is actually being used for.
When years, months, and days is the right format
For identity, forms, eligibility rules, and everyday conversation, age in years (sometimes with months) is the universal convention — it's what people expect, and it's what most official documents and systems are built around.
When a total-day count is actually more useful
A total-day count matters for milestone tracking (someone's literal 10,000th day alive), for precise duration comparisons that don't respect calendar-year boundaries (comparing two people's exact tenure down to the day), and in some medical or scientific contexts where age in days is the standard unit, particularly for infants.
It also avoids a subtlety that years-based age has: two people can be the "same age" in years while differing by up to 364 days in actual time lived, depending on exactly where their birthdays fall relative to a comparison date.