In a tax return, credits related to dependents generally include the dependent's age in determining whether or not the taxpayer qualifies for the credit. Additionally, knowing when an individual in a return has reached age 65 is important for several purposes.
For an individual born on January 1, it's important to know their age for tax return purposes since the tax year for individual taxpayers ends the day before January 1.
For a taxpayer born on January 1, their age at the end of
the tax year (December 31) equals their age the next day.
Thus you can use this formula to determine the age of an individual born on January 1.
Tax Year minus Birth Year + 1
For example, an individual born January 1, 2005 is considered age 19 at the end of tax year 2023 because
2023 - 2005 + 1 = 19