The thesis explores the detection and characterisation of exoplanets orbiting young stars (<=1 Gyr), offering key insights into the early stages of planet formation and atmospheric evolution. By combining transit photometry and radial velocity data from both space- and ground-based observatories, we investigate how features like the radius valley and the Neptunian desert emerge. A major challenge addressed is the impact of stellar activity, which is mitigated using Gaussian Processes to isolate planetary signals.

We characterise a range of young planets, including intermediate-age sub-Neptunes (HD63433 b, c, and d, TOI-1801 b, TOI-1687 b) and Saturn-sized planets (TOI-1135 b), as well as very young Neptune- and Jupiter-sized planets such as AU Mic b and c, K2-33 b, TOI-837 b, and HIP 67522 b and c. These cases highlight signs of atmospheric inflation and ongoing photoevaporation. A key result is the first large compilation and population analysis of transiting young exoplanets, which could reveals that the youngest exoplanets show a distinct radius distribution compared to older systems. Overall, this work provides important empirical constraints on models of planetary formation and early evolution.