Your age is a convention

The number you call your age is a counting convention, not a measurement. Traditional Korean age counted a newborn as one and added a year every 1 January, so a baby born on 31 December was two years old the following day while being one day out of the womb. Chinese nominal age still works that way against the Lunar New Year. And a year is one orbit, so a 36-year-old is 149 on Mercury and 19 on Mars.

The international count — completed years since birth — is one convention among several, and it is recent enough that South Korea only made it the legal default in June 2023.

How did the Korean system work?

You were one at birth and gained a year at every New Year, not at every birthday. That produced the situation the reform ended: three age systems coexisting in one country, with traditional age used socially, international age used legally in some contexts, and a third "year age" — the current year minus the birth year — used for conscription and school entry.

Someone could genuinely have three different correct answers to how old they were, depending on who was asking. The June 2023 change made the international count the default for legal and administrative purposes, though the traditional count survives in conversation.

What is Chinese nominal age?

The same idea against a different calendar. Nominal age — 虚岁 — counts a newborn as one and adds a year at each Lunar New Year, so it runs one or two years ahead of the international count depending on whether the New Year has passed.

The underlying idea is different rather than wrong: the number states the year of life you are currently in, not the number of years completed. It is the same logic as calling the first year of a century the first, not the zeroth — ordinal rather than cardinal counting.

That is worth holding onto, because it explains why the systems feel arbitrary from inside the other one. Neither is counting badly; they are counting different things.

What happens to a year on another planet?

It changes by a factor of six hundred across the solar system, because a year is one orbit and orbits vary enormously.

Days are stranger than years. A day on Venus lasts 243 Earth days — longer than its 225-day year — so a Venusian day is longer than a Venusian year, and the Sun rises in the west because Venus rotates backwards.

Rotation direction is not the only oddity. Uranus is tipped on its side by 98 degrees, so each pole spends about 42 Earth years in continuous daylight and 42 in darkness — a Uranian year is 84 Earth years, and half of it is one long day at each pole.

Why does any of this matter?

Because thresholds are written against a convention, and the convention has to be stated. A drinking age, a school entry date, a pension age and a conscription list can all be reading different numbers from the same birth date if the systems in use differ.

It also matters for anything international. A form asking for age rather than date of birth is asking a question with more than one correct answer, which is why date of birth is what official systems collect.

Two more systems are worth a mention because they turn up in records. East Asian year age — the current year minus the birth year — ignores the birthday entirely and is what Korean conscription and school entry used. And astronomical Julian day numbers count days continuously from a fixed point in 4713 BC, which is a date system with no months or years in it at all.

What survives across all of them?

The birth date itself, and the day count. Both are facts rather than conventions, which is why they are what gets recorded and why every age system is derived from them rather than the other way round.

That is the practical answer to any cross-system confusion: exchange dates, not ages. A date can be converted into whichever count the other side uses; an age cannot be converted back without knowing which count produced it.

Questions people ask

Is Korean age gone entirely? Legally, yes, since June 2023. Socially it persists in conversation, which is the usual fate of a counting habit that a law has changed.

Does Chinese nominal age still get used? Yes, particularly in traditional and family contexts, alongside the international count for official purposes.

Why is a Neptune age so small? Because one Neptunian orbit takes nearly 165 Earth years, so nobody has ever been one Neptune year old. Neptune has completed one orbit since its discovery in 1846.

What is a sidereal versus a solar day? The sidereal day is one rotation relative to the stars; the solar day is one relative to the Sun, and it is slightly longer because the planet has also moved along its orbit. On Earth the difference is about four minutes.

Exchange the date rather than the number, and every system can derive its own answer. The Korean age and Chinese age calculators convert between the counts, and age on other planets does the same thing with orbits instead of calendars.