Chronological age (CA) represents the exact time elapsed since a person’s birth up to a specific date. In occupational therapy, determining CA is foundational for administering standardized assessments to children. These assessments require comparing a child’s performance against age-matched peers. An accurate CA ensures the child is measured against the correct developmental group, making the results meaningful for intervention planning.
Establishing the Calculation Format
Calculating chronological age begins by setting up two required dates in a standardized structure. The first is the Date of Test (DOT), the day the assessment was administered. The second is the child’s Date of Birth (DOB). Both dates must be formatted using the Year, Month, Day structure (YYYY/MM/DD). The Date of Test (DOT) must be placed directly above the Date of Birth (DOB) to ensure the subtraction proceeds correctly.
Calculating Chronological Age (Simple Subtraction)
When the days and months of the Date of Test are numerically larger than those of the Date of Birth, the calculation is straightforward. The subtraction process must be performed from right to left, beginning with the days column. For example, if the DOT is 2025/08/20 and the DOB is 2020/05/15, no adjustments are required. Subtracting the days (20 minus 15) yields 5 days, the months (8 minus 5) yields 3 months, and the years (2025 minus 2020) yields 5 years. The final chronological age is 5 years, 3 months, and 5 days.
Calculating Chronological Age (Handling Borrowing)
The calculation becomes complex when the day or month in the Date of Test is numerically smaller than the corresponding value in the Date of Birth. In these instances, “borrowing” must be used to adjust the numbers before subtraction. If the DOT days are smaller than the DOB days, borrow one month from the months column. This borrowed month is converted to 30 days and added to the DOT day column for standardization. If the DOT months are smaller than the DOB months, borrow one year (12 months) from the years column and add it to the DOT months.
If the DOT is 2025/05/10 and the DOB is 2020/08/15, borrowing is required for both days and months. Since 10 is less than 15, borrow one month (30 days) from the 5 in the months column, making the new day count 40, and the months column is reduced to 4.
Since the new month value (4) is less than 8, borrow one year (12 months) from the years column, making the new month count 16. The year column is reduced to 2024. The final subtraction (2024/16/40 minus 2020/08/15) results in a CA of 4 years, 8 months, and 25 days.
Applying Chronological Age in Standardized Assessments
The final chronological age is essential for accurately interpreting a child’s performance on norm-referenced assessments used in occupational therapy. Once the age in years, months, and days is determined, therapists use this figure to locate the appropriate normative tables within the assessment manual. Assessments such as the Peabody Developmental Motor Scales (PDMS-2) and the Bruininks-Oseretsky Test of Motor Proficiency (BOT-2) rely on CA to establish the comparison group.
The CA directs the therapist to the specific manual section that provides standard scores, percentile ranks, and age equivalents. If the chronological age is calculated incorrectly, the therapist might use the wrong normative table, leading to an inaccurate interpretation of the child’s abilities. An accurate CA is foundational to determining if a child’s skills fall within the expected range, guiding the development of appropriate intervention goals.