Same as any other scholarship program...Walker came in under the old NSW scholarship, so was bound to the club prior to hitting draft age (18). Wasnt able to find anything definitive on whether or not that allowed them to train with the club in the years before the draft, but my recollection was that they did.
Not much detail, but is mentioned here: https://www.theroar.com.au/2008/12/...cholarship-holders-and-their-path-to-the-afl/
... and Hird was just a plain old draftee.