The characterization of young planets (<300Myr) is pivotal for understanding planet formation and evolution. We present the 3-5um transmission spectrum of the 17Myr, Jupiter-size (R~10R_{Earth}_) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous g-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%-50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b's mass is <20M_{Earth}_ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8+/-1.0M_{Earth}_. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10g/cm^3^, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H_2_O and CO_2_ (>=7{sigma}), a modest detection of CO (3.5{sigma}), and weak detections of H_2_S and SO_2_ (~2{sigma}). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3x10 solar due to the amplitude of the SO_2_ feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01-0.03M_{Earth}_/Myr), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100Myr of its existence.