From what I've observed it would seem to depend somewhat on the quality of player/s available in each age group, who takes up the offer of a scholarship and which World Championships is being held in that year, either U17 or U19.
Don't think there's too much science to it, it sorts itself out based on some of the above criteria.
Believe it or not some kids get offered scholarships, but don't go for a variety of reasons.
Most that go will be there for 2-2.5 years, some 3.5 years. eg Dante had 3.5 years
The odd one will only have one year, more likely to be 6-7 months, before they go to college in the US. These are generally the late bloomers who are filling in time between finishing Y12 & going to College, but some are kids who haven't gone earlier due to being at a good school, education wise, family reasons, etc. (ie. This year Gerard Martin, Xavier Cooks, last year Dane Pineau, Felix Von Hofe). Gerard was there 1.5 or 2.5 years, Xavier, Dane & Felix only 6 months or so. Xavier was a late bloomers, Dane was a bit of a mix between late bloomer & valued his private school education, Felix don't know, but went to a good private school in Melbourne.
This past year the majority where eligible for the U17 World Championships (7 of the 12), 2 departed half way through to go to College, the other 3 are doing Y12 this year and expect them to be back next year as there is possibly an U19 Worlds.
So I'd expect any new additions to be older, either doing year 12 next year or completing year 12 this year.
This time next year they'll probably go for some Y10 (maybe if there is some exceptional talent, or any 6'9" + 15 or 16yo) or Y11 kids in readiness for the 2016 U17 World Championships.