If you are familiar with MIS (the church's membership database), you'll notice that when they reach the age of eight they are removed from membership roles if they are not baptized within their eighth year. However, if their parents are still members, their names are kept in MIS (not as members, but part of a family who has a member) with the parents until they are 18. Non-Baptized spouses are also included. If I remember the designations for these are "Child of Record," (0-8) "Unbaptized" (9-18) and "Non-Member" (18+). They used to have an "unaccountable" designation as well for those with disabilities who don't need to be baptized, although I've not seen it recently (no "unaccountables" in my last ward that I'm aware of), and they'd also be included with "Child of record" in my opinion. Church statistics are more accurate than most other denomination's "estimates" and the computer system designates and distinguishes quite easily. The Catholic Church for example bases theirs on country statistics - if a country is deemed "catholic" then 90+ percent is counted as being catholic - even if they are not, or if they have had their names removed. See this recent thread  and resulting discussion. -