Understanding costs and effectiveness of mechanical fuels treatments is essential for efficiently influencing fire behavior at the landscape scale. While the capability of mastication to mitigate the potential of high severity wildfire in mature stands has been studied, the variables that determine its efficiency across age classes have received far less attention. The objective of this study was to examine how stand age influenced the efficiency and effectiveness of mastication. Mastication was performed on stand ages and structures representative of those commonly observed in the mixed conifer forest of the Sierra Nevada. Machine data from spatial and motion sensors were used to assess machine efficiency, and fire modeling was used to evaluate treatment effectiveness. The stands masticated were 10, 33, and 100 years old. Machine operation time per hectare was shown to vary with stand age, with the 10 and 100-year-old stands less costly to treat than the 33-year-old stand. However, potential for high severity wildfire immediately following treatments was reduced to a much greater degree in the 33-year-old stand, whereas it increased for the 10-year-old stand and was unaffected in the 100-year-old stand. This study's findings suggest that masticating developing stands (i.e. those entering the stem exclusion phase), while relatively costly, may also provide managers with a big “bang for their buck” in reducing fire severity across the landscape. To assess longer-term effectiveness, it will be important to track predicted fire severity as masticated fuels decompose and as tree growth responds to the reduction in density following the treatments in stands of various ages.