Additively Manufactured (AM) metastable β-Ti alloys are strong candidates for aerospace and biomedical applications due to their potential for exceptional strength through precipitation hardening. However, post-AM heat-treatments and machining operations are typically required to achieve the desired properties and design tolerances, necessitating careful sequencing considerations of these processes. This study investigates the machinability of Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C) produced by Wire Arc Additive Manufacturing (WAAM) under Solution-Treated (ST) and Solution-Treated and Aged (STA) conditions. Results show that ageing significantly impairs machinability, increasing cutting forces by over 20%, accelerating tool wear, and degrading surface finish. Elevated cutting temperatures cause cobalt binder depletion and rapid tool deterioration. Tool life is halved after ageing, emphasizing the need to optimize heat-treatment and machining sequences when manufacturing components.