Understanding Society User Support

I am attempting to merge individual files from harmonised bhps and ukhls into long format. The datasets I am using are the special license datasets 6931. In the syntax provided in the documentation page, it uses the age_dv (i.e., derived variable). However, it seems that these dervide variables are not present in the BHPS indresp stata files. Could someone enlighted me as to whether I am missing something? Or perhaps, how to derive such variables myself?

Many thanks