You are apparently talking about age at immunization, rather than age. Age in months is given as hw1 for all surviving children age 0-4 years. The KR file includes children born in the past 5 years who died, as well as those who survived. If the child died (b5=0) then hw1 is missing and there is also no immunization data.

Age at immunization can only be calculated if we have the date of immunization, which requires a health card with a date. That's given as h7=1. If h7 has any other code, then h7d, h7m, and h7y will be missing and you cannot calculate date of immunization or age at immunization.

There are other children who are taken to be immunized because the mother said so, but for them, no date is recorded. If there were a date, it would not be reliable.

I will paste below some Stata lines for calculating age at immunization, in months, for those children with non-missing date.

set more off
tab1 h7*

gen h7d_rev=h7d
replace h7d_rev=. if h7d>31

gen h7m_rev=h7m
replace h7m_rev=. if h7m>12

gen h7y_rev=h7y
replace h7y_rev=. if h7y>v007

gen h7_mdy=mdy(h7m_rev,h7d_rev,h7y_rev)

gen dob_mdy=mdy(b1,hw16,b2)

* "exact" age in months; the average days in a month is 365.25.12 = 30.4375
gen h7_age_in_months=(h7_mdy-dob_mdy)/30.4375
summarize h7_age_in_months 

* completed age in months
gen h7_completed_age_in_months =int(h7_age_in_months)

summarize h7_age_in_months
tab h7_completed_age_in_months