  Young transiting exoplanets offer a unique opportunity to characterize the
atmospheres of fresh and evolving products of planet formation. We present the
transmission spectrum of V1298 Tau b; a 23 Myr old warm Jovian sized planet
orbiting a pre-main sequence star. We detect a primordial atmosphere with an
exceptionally large atmospheric scale height and a water vapour absorption at
5$\sigma$ level of significance. We estimate a mass and density upper limit
(24$\pm$5$M_{\oplus}$, 0.12gm/$cm^{3}$ respectively). V1298 Tau b is one of the
lowest density planets discovered till date. We retrieve a low atmospheric
metallicity (logZ=$-0.1^{+0.66}_{-0.72}$ solar), consistent with
solar/sub-solar values. Our findings challenge the expected mass-metallicity
from core-accretion theory. Our observations can be explained by in-situ
formation via pebble accretion together with ongoing evolutionary mechanisms.
We do not detect methane, which hints towards a hotter than expected interior
from just the formation entropy of this planet. Our observations suggest that
V1298 Tau b is likely to evolve into a Neptune/sub-Neptune type of planet.
