I was curious about this one, too, when I read it on a few...and I think not just from Uganda.
According to a survey of the Ugandan education system by the European Union, there are many stages of schooling:
Preschool
Primary Education
1st Cycle of Secondary Education (general education)
2nd Cycle of Secondary Education (vocational or technical education)
Technical colleges
Universities
Non-formal institutions (I presume these are much like undertaking an apprenticeship with the carpenter down the road??)
(
http://www.uis.unesco.org/template/pdf/scb/StateReports/InformationAndMonitoringSheet/Uganda.pdf)
Wikipedia tells me that primary school lasts for 7 yrs, 1st cycle secondary for 4 yrs, 2nd cycle secondary for 2 yrs, & post-secondary education usually lasts 3 to 5 yrs. Only around 40% of kids attend primary school, it's not too common to go on to secondary schooling, & upon secondary graduation, it can be very difficult to gain entry in to further education (
en.wikipedia.org/wiki/Education_in_Uganda)
http://education.stateuniversity.com/pages/1585/Uganda-EDUCATIONAL-SYSTEM-OVERVIEW.html:
Kids start primary school (not preschool) at age 6yrs - this quotes primary school attendance at about 80%, but agrees that not so many go on to secondary education (about 25%). "Ugandan females are classified as a "disadvantaged" group, along with orphans, migrants, poor students, and the disabled", so enrollment of girls is somewhat less common, though increasing due to public policy & changing social attitudes.
Soooo....if a kid starts school at 6yrs, they should finish primary school by about 13yrs, 1st cycle secondary by 17yrs, 2nd cycle secondary by 19yrs, & will probably be done studying at the university or other higher level by age 22-25yrs. I guess this gels pretty well with the Western education system??
As related to Kiva entrepreneurs....I guess it's possible that some of the children in their early-20s are still in school, but my guess is that when a write-up states that all children are in school, that this refers in fact to all children who are of school going age, rather than
all of them. This also fits with a couple of write-ups, I think from Uganda, that I've seen saying that the entrepreneur has children aged, for example, 1, 5, & 8yrs, & all are in school. I always took this to mean that the 5 & 8yr olds are in school & the entrepreneur has the intention of sending the 1yr old to school when s/he is of age, too.
Sorry to ramble, hope this answers your question a bit!
Peace to you.