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Say a person was born at noon on January 1st, 2000. Under US law, what is the exact time that person legally turns 18? Is it the midnight before January 1st 2018, or the midnight after, or noon, or something else?

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In the US, the common law rule is that a year of age is completed on the day preceding the anniversary of one's birth.

Here are some citations from cases that relied on this rule:

  • "Full age in male or female is 21 years, which age is completed on the day preceding the 21st anniversary of a person's birth." State in Interest of FW 130 N.J. Super. 513 (1974) (citing Blackstone)
  • "The common law rule for computing age is that one is deemed to have reached a given age at the earliest moment of the day preceding an anniversary of birth." Fisher v. Smith, 319 F. Supp. 855 (D. Wash. 1970)
  • "The appellant did, then, reach the age of nineteen years on the day before the nineteenth anniversary of his birth" Turnbull v. Bonkowski, 419 F.2d 104 (D. Alaska 1969)

This is codified in some statutes and regulations. For example, CFR 404.102:

For the purpose of this subpart [...] You reach a particular age on the day before your birthday. For example, if your sixty-second birthday is on July 1, 1979, you became age 62 on June 30, 1979.

In Virginia, the Attorney General has confirmed that the common law rule is in effect:

a person attains his/her next year of age on the day prior to his/her birthday

In your example, barring a modification by state law, or an idiosyncratic treatment under a particular statute, the person would be deemed 18 years of age at the earliest moment of December 31, 2017.


For a bit of comparative law, contrast with the treatment in British Columbia:

A person reaches a particular age expressed in years at the start of the relevant anniversary of his or her date of birth. (Interpretation Act s. 25)

  • do you also know how the different time zones play into it? Example: illegal phone call from NY to LA :) – user3232 Oct 28 '15 at 3:51
  • Correct me if I'm wrong, but this doesn't affect drinking at the bar, buying cigarettes, etc. I wonder why the schism... – Brad Werth Oct 28 '15 at 6:43
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    Wow, that's very interesting. I just assumed it, because where I am from they have these little signs by the door or register that say "you must be born before this day/year to buy...". I doubt one would be able to convince many establishments otherwise. Now I'm curious about whether they have a legal requirement to serve someone in that situation... I might need to make a new question to explore this further. Thanks! – Brad Werth Oct 30 '15 at 16:13
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    Now I'm curious about whether they have a legal requirement to serve someone in that situation Typically, you can refuse service to anyone for any reason, as long as you aren't discriminating against a protected class. "People whose birthdays are tomorrow" are not a legally protected class in the US and stores would be within their rights to not even let such people inside, let alone serve them anything. Then again, age discrimination is illegal and I suppose one could argue that rule disproportionately affects 20-year-olds. – Patrick87 Oct 30 '15 at 20:52
  • @user3232: First, your birthday would be based on the time zone where you are born. An Australian could be born 18 hours before an Alaskan and have a later birthday. And for that illegal call from NY to LA your age would depend on where it goes to court. At the right place, you might be able to drink legally, walk 20 meters to a bar in another state where the time is snow hour later, and drinking alcohol might be illegal for you. – gnasher729 Nov 30 at 17:31
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It is logically simple why you turn the legal age the day before your birthday. If you were born in January 1, 2000 (like the example above), on December 31, 2020 you have been 'alive' for exactly 21 full years. (the year 2000 plus 20 more years). I have a unique reason I know this to be true. I was born on November 9th, 1998. The day before in 2016 (November 8th) happened to be the 2016 Federal elections in the US. I thought I had missed it by ONE day of being able to vote, but my brother is an attorney and did some enquires and found out that I was legally 18 (in California) the day before my birthday. So I got to vote!!! (NOT happy with the results btw, lol).

Finally this applies for US Federal candidates as well. Congress is sworn in on January 3rd, so a person must be 25 (for the House of Representatives) or 30 (for the Senate) to be sworn in. Thus a person born on January 4th 1996 is eligible to run for the US House in the 2020 elections, as they would be legally assumed to be 25 on January 3, 2021 (the day they are sworn in and the day they are legally 25). The US president is sworn in on January 20th and must be 35 years old, so a person a person born on January 21, 1986 could actually run in the 2020 race, as they would be legally 35 years old on January 20, 2021.

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    I still don't understand the logic. If you were born at, let's say, 3:15 am on on January 1, 2000, then the instant at which you have been alive 21 years is 3:15 am on January 1, 2021 (ignoring leap years and time zones). At no time on December 31, 2020, have you been alive for 21 years. – Nate Eldredge May 25 at 17:36
  • US Senator en.wikipedia.org/wiki/Rush_Holt_Sr. was nominated, ran and was elected while 29 years old. He was not allowed to sit in the Senate until his 30th birthday, in June... – DJohnM May 25 at 18:22
  • @NateEldredge when the common law rule was developed, it was neither customary not possible to measure time that precisely. I agree however that it is illogical. Still, logical or not, it is the way of calculating age that prevails in many jurisdictions. – phoog May 26 at 7:41
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    @NateEldredge the law takes the view that "a day begun is a day done". If you have been alive at any moment of the day then as far as the law is concerned you have been alive for the entire day as the law operates in units of one day. – A. K. May 27 at 17:05
  • @A.K.: Aha, that makes more sense. So in the above example, as of any moment on December 31, 2020, there are 365*21 days on which you have been alive for at least one moment (again ignoring leap years). This includes two days on which you weren't alive for the entire day: January 1, 2000 (because you were born partway through the day) and December 31, 2020 (because the day is not yet finished) - but they are counted anyway. – Nate Eldredge May 27 at 17:28

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