Dear user,
You would need to use the BR or KR file and code the variable to obtain the number in the following way. BR is all births and KR are births in the last five years. I call the variable child420 which gives the number and percentage of children born before the mother was age 20.

Stata code:

gen agem=b3-v011
gen agemyr = int((agem) / 12)
gen childb420=0
replace childb420=1 if agemyr<20
ta childb420

Thank you.

Best,