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?
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)
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.