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Demographic studies exoplanets have revealed population level trends, such as the exoplanet 'radius valley'. A widely accepted hypothesis to explain these demographic features is that exoplanets on compact orbits undergo early evolutionary processes (such as mass loss) over the first few 100 million years of their life, which is expected to significantly alter their atmospheric properties. Young transiting planets (< 100Myr old) represent the earliest phase in the lifetime of exoplanets and are promising targets to reveal their evolutionary history. However, it is not known how the primordial atmospheres of these planets look like: What is nature and composition of their atmospheres? How diverse are the atmospheric properties right after formation? How do their atmospheres compare with their mature counterparts? To address these questions, in this talk we will present the first observations of the atmospheres of young transiting planets with the Hubble Space Telescope and the James Webb Space Telescope. We characterize the atmospheres of three young planets (20-30 million years old) to measure their mass, atmospheric composition and internal entropy using transmission spectroscopy. We compare them with each other as well as with the mature exoplanet population to understand the impact of early evolutionary mechanisms on their atmosphere. Our findings challenge predictions about the atmospheric composition and internal entropy from the standard core-accretion planet formation and thermal evolution models. We explore new ideas about the internal structure of these planets which could potentially reconcile them with their mature counterparts.