Population age needed for research does not line up with age groupings in census data



So I am looking to find the population for a County and MSA in the 18-29 age group.

However, census data groups age by 15-19, 20-24, and 25-29. I am good to go in the 20-29 age range, but the 15-29 group is what is throwing me off...

Is there a credible way to infer a subset of the population group 15-19 for strictly 18-19 age group in order to get a reasonably accurate measure of this age grouping?

Thanks in advance!