...and it just so happens that exactly 64,696 are born each month, how many babies are aged between birth and 9 months at any given time?
Approximately $\dfrac{9}{12}776352 = 582,264$. This assumes an even distribution of births throughout the month (meaning births happen more frequently in shorter months than they do in longer ones). It also assumes that you consider a baby to be within the range up to the second before exactly nine months passed since their birth and no longer in that range exactly one second after nine months after their birth.
I apologize. It actually considers a "month" to be exactly $\dfrac{1}{12}$ of a year rather than an actual month. Otherwise, the number will fluctuate throughout the year depending on how many days are in the preceding 9 months.