Following is a response from Senior DHS Staff Member, Tom Pullum:
The following Stata lines will calculate the age of the youngest child in a way that is consistent with b8. Here "age" is not restricted to children who survived but is completed years since birth for all children. When you compare with b8_01, that comparison will automatically be just for the children who survived, because b8_01 is NA for children who died. Here is no need to condition on the value of v208; the calculation applies for any value of v208. Here, the last line restricts to the youngest child under 5. It shows that "age" and b8_01 agree exactly.
In a previous post you used "round" in place of "int". Here, as in that context, you need to use "int" to get completed years of age.
* Calculate the age of the most recent child in the IR file
use "...IAIR74FL.DTA", clear
gen age=int((v008-b3_01)/12)
tab age b8_01 if age<=4