Implicit within the second half of the Rashi comment on 30:16 is a rather interesting implication of ages were determined for the census, and thus for military service as well.

Please note where Rashi explains that the age of a young man who was 19 at the first census in Tishrei, and the second in Iyar.

But the years of the men are counted by the reckoning of the years of the world since Creation, which begin from Tishrei.

I find it surprising to see this small fact mentioned here and not at the command discussing the age of army or Levite service.

By the way we seem to measure our ages, each person knows their birthday, and counts their number of birthdays. And I have never thought that the Torah meant other than that for the Israelites' service.

But if that were the case for them, then this Rashi comment would not make any sense, since some fraction of the 19 year old men would have had their 20th birthday between the two countings.

This Rashi teaches us that apparently they measured their number of years simply by the difference between the current year and the year they were born, or counting the number of Rosh Hashana's they have seen.

Thus their number of new-years they have seen would remain the same from one Tishrei until the next, and a birthday between Tishrei and Iyar would be irrelevant.

This reasoning would probably not go over very well for a young man in a state with a drinking or driving age minimum the same as the number of New Years he has seen if he still has a few months to go by his birth date.

But we do see this type of grouping by cut-off date (quantization) of age for enrollment to public schools, where the school year starts on a certain day, and some day close to that is the arbitrary cut-off day, which can vary from school district to school district, for which children can or should enroll into their first year of school.

We don't for example say that a 5 year old child should attend their first day of kindergarten at school on the day after their birthday... that if they have less than a month to go in the school year, that they will end up with less than a month of kindergarten, then will start 1st grade the next fall.

Now military training such as a 6 week boot camp program pretty much runs year round, I think, in the US. A new recruit in any month will attend a training program based on when they enlist. And in a time when young men are eager to enlist, but must wait for a certain age, they might in fact enter basic training right after their birthday.

Perhaps the Israelite military had a training period. Perhaps it was held only once per year. It would then make sense to have a cut-off period for birthdays, and treat each person based on what was their age as of Rosh Hashana.

This concept of quantized age would probably not work very well for the way we celebrate Bar Mitzvah these days either.