Why does under the age of 22 matter?
If the numbers overall have increased since 1979 then how does that not speak for itself? Unless there is anything to suggest that the age of apprentices has changed (and there isn't).
From my source:
"In 2008, the definition for apprentices and trainees changed from those employed as apprentices/trainees to include only those with a formal contract under the Australian Apprenticeship Scheme. Therefore data on apprentices from previous years are not directly comparable to 2008 and subsequent data. Note that Australian School-based Apprenticeships are excluded."
From the glossary of the 2012 numbers - "An apprentice is a person aged 15–64 years who has entered into a legal contract (called a training agreement or contract of training) with an employer, to serve a period of training for the purpose of attaining tradesperson status in a recognised trade. In this survey, apprentices are identified by their answer to a question specifically pertaining to the Australian Apprenticeship Scheme. Note that Australian School-based Apprenticeships are excluded"
In short the numbers are probably slightly understated.
If you think that definition doesn't encompass your concerns then can you clarify how it doesn't?