How to Calculate Someone's Age from Their Date of Birth

How old is someone born on June 15, 1990? As of today (June 15, 2026), they're exactly 36 years old. Simple.

But what if their birthday is February 29? What if you need their age in months for a medical record? What if you need to know whether they've reached a specific age cutoff for eligibility?

Here's how age calculation works in various contexts.

Basic Age Calculation

Age in years = Current year - Birth year (adjusted if birthday hasn't occurred yet this year)

Example 1: Born March 10, 1990. Today: June 15, 2026.

2026 - 1990 = 36. Birthday (March 10) already passed in 2026. Age: 36 years

Example 2: Born September 20, 1990. Today: June 15, 2026.

2026 - 1990 = 36. Birthday (September 20) hasn't occurred yet in 2026. Age: 35 years

Age in Months

Age in months = (Years × 12) + months since last birthday

Born March 10, 1990. Today: June 15, 2026.

36 years × 12 = 432 months

Months since March 10: March → April → May → June = 3 complete months

Total: 432 + 3 = 435 months

Age Cutoffs: The "As Of" Problem

Many eligibility determinations use age as of a specific date, not the current date:

• School enrollment: Must be 5 years old by September 1
• Sports leagues: Age as of December 31 of the competition year
• Retirement: Age as of the first of the month
• Medicare: Eligibility beginning the month you turn 65

The formula: Age as of [specific date] = apply the birthday-has-passed rule using that date instead of today.

The February 29 Birthday

People born on February 29 (leap day) have a birthday that only appears on the calendar every 4 years. How is their age calculated in non-leap years?

Legal approaches vary by jurisdiction:

• Most countries: March 1 is the recognized birthday in non-leap years
• Some: February 28 is recognized
• UK: February 28 for most purposes

For everyday calculations, most people celebrate on either February 28 or March 1 in non-leap years. For legal age milestones (voting, drinking, retirement), check the specific law in your jurisdiction.

Age Verification in Practice

Age verification for legal purposes (alcohol purchase, voting, age-restricted content) uses the exact birth date. Being one day short of 18 is legally the same as being 17.

This is why precise age calculation matters — and why systems that check age use exact date comparison rather than estimated age.

Calculate anyone's exact age from their birthdate with our Date Calculator.