The birth years of people are considered to be measured at the interval level. This is because the years are numerical values that indicate the temporal order of birth, and the difference between any two years has meaningful interpretation (i.e., the difference in years indicates an actual time difference).
While birth years do not have an absolute zero point in the same way that ratio levels of measurement do (e.g., height or weight), they are still meaningful in terms of ordered intervals. Thus, the correct classification for this data is interval measurement.
It's important to note that birth years are not nominal (categories with no inherent order) or ordinal (categories with a meaningful order but not equal intervals). They also cannot be classified as integral (which refers more to whole numbers and counts rather than a specific level of measurement).
In summary, birth years are best classified as interval level measurements.