Following is a response from Senior DHS staff member, Tom Pullum:
The following Stata lines will construct this three-category version of the mother's age at the birth of the child. You should be able to translate it into SPSS. The function int(x) constructs the integer part of a number. For example, int(1.1) is 1, and int(1.7) is also 1. There should be something similar in SPSS.
* Mother's age at the birth of the child
replace moage=2 if moage_years>=20
replace moage=3 if moage_years>=35
label variable moage "Age at birth"
label define moage 1 "<20" 2 "20-34" 3 "35+"
label values moage moage
tab moage [iweight=wt]