Your interpretation of these variables appears correct to me. FCDAGE_ALL reports the ages at cutting of each of the respondent’s daughters. The availability tab for this variable notes that it is provided for 15 countries in total, including for 11 Central and West African countries during survey phases 6 & 7. While FCAGEDAU is available for samples in survey phases 3-5, you are correct that this variable alternatively reports the age at cutting of the woman’s eldest daughter or the age at cutting of the most recently cut daughter. The universe tab for this variable provides a helpful overview for whether the sample refers to the former (universe statements that include “whose eldest living daughter is circumcised”) or latter (“with at least one living daughter who is circumcised”) definition. Regarding the Nigeria 1999 sample, the DHS Program notes that it “was not centrally involved in this survey and cannot stand behind the quality of this survey.”
You might create a comparable measure across these samples that reports the age at cutting of the respondent’s eldest daughter. For FCAGEDAU, the variable FCDLINENUM reports the line number of the most recently cut daughter. This variable can be used together with HHAGE on the household member’s unit to determine whether the age reported in FCAGEDAU refers to the eldest daughter or to a younger daughter. A consistent measure for FCAGEDAU would apply the same universe across samples by reassigning a code of 99 (“NIU”) to all households where this variable refers to a younger daughter.