Quick answer

At 12:00 AM on their birthday , a person is considered one year older (legally and socially), even if they were born later in the day (like 10:13 AM).

What’s actually going on?

There are two ways to look at age :

1. Legal / everyday age (what people use)

  • Your age increases at midnight at the start of your birthday.
  • So at 12:00 AM , you are officially your new age.
  • Example: You turn 20 the moment the clock hits midnight.

2. Exact chronological age (precise timing)

  • Technically, you haven’t completed the full extra year until your birth time.
  • So between 12:00 AM and 10:13 AM , you are:
    • 20 years old (legally),
    • but 19 years + ~23 hours + 47 minutes × number of years lived in exact time terms.

Simple example

If someone was born at 10:13 AM on July 7, 2000 :

  • At 12:00 AM on July 7, 2026 :
    • Legal age: 26
    • Exact age: 25 years, 364 days, 13 hours, 47 minutes
  • At 10:13 AM on July 7, 2026 :
    • Legal age: 26
    • Exact age: exactly 26 years

Why we use midnight

  • It keeps things simple for society (IDs, laws, celebrations).
  • Tracking exact birth times for everyone would be impractical.

Bottom line

  • Midnight = official age change
  • Birth time = exact biological age change

TL;DR

At 12:00 AM on your birthday, you are officially your new age—even if you weren’t born until later that day.