Since the inception of the Youth Criminal Justice Act (YCJA) 20 years ago, the Supreme Court of Canada (SCC) has considered the adult sentence applications framework on only one occasion in R v DB, 2008 SCC 25. Additionally, Parliament enacted a major legislative overhaul of the provisions related to adult sentence applications in 2012. Without further input from the SCC, trial and appellate courts across Canada have filled the gaps in interpreting section 72 of the YCJA. On November 23, 2023, the SCC has announced they will hear the appeals of R v IM, 2023 ONCA 378 and R v SB, 2023 ONCA 369 and revisit the issue the presumption of diminished moral blameworthiness. The adult sentence application provisions are arguably the most important provision within the Act, given the serious jeopardy that goes along with an adult sentence. Courts across Canada have struggled to give constitutional effect to the age-based presumption of diminished moral blameworthiness. In this thesis, I present a critical examination of the current approaches to adult sentence applications and outline the best practices for youth court judges, Crown attorneys and defence lawyers. In support of this end, I have conducted a survey of 20 years of trial level adult sentence decisions from across Canada tracking the age, race, gender, support structures, and cognitive and mental health challenges, among other factors. I have framed this inquiry with the Critical Legal Studies tradition. However, given that adult sentence applications touch on a broad range of complex issues, portions of this thesis engage other legal and interdisciplinary theories including Critical Discourse Analysis, Law and Emotion Theory, Critical Race Theory and Disability Legal Theory. Ultimately, this thesis highlights the importance of respecting the letter and spirit of the law set forth by Parliament and by the Supreme Court of Canada in DB, in anticipation of the SCC’s upcoming consideration of these very important principles