Um, if you're 16 or 17 years old, you're still a junior. Unless you're inferring that kids who aren't good enough to play U15 or U16 TAC Cup miss out on U18 (which is not always the case). If you think about it hard enough - the TAC Cup IS the VFL U18 competition.
The problem they have is with the draft age, and the junior state comp (TAC) being for U18s instead of U19s or even U20s. One of the biggest mistakes they made was abolishing the VFL U19 comp and dropping the maximum age for TAC. If you watch VFL reserves they're invariably an U21 side, pretty skilful but physically shy of full development. This is where the 'late bloomers' are expected to sprout from, progress to the VFL senior side and play against AFL-listed players.
Frankly, if the minimum draft age was 19, then pretty much every kid would be forced to study hard to get into uni and do their first year of study, or commence an apprenticeship, or work full-time for a year, before they were given the nod to play professional footy. This would assist with ensuring they had something to fall back on (if they were in the 85-90% of players who get cut within 4 years). Remember, we aren't still playing in the 1970's when Tim Watson could rock up and play senior footy at 15 years of age.
We should be looknig to create a system akin to the NBA College system where (with the odd exception of a Kobe Bryant or LeBron James) every player who ends up in the big league has spent 3-4 years at university. Given we don't have the same longevity in AFL footy, it should be reduced to 2 years post high school. That's when you know if a Hodge will be a Judd, etc.
Also - no-one would be whacking off about N Ablett until he turned 19 or 20, same as all these gun TAC juniors, several of whom will never be able to translate their game into AFL style footy.