Despite thousands of extrasolar planets discovered to date, the dominant mechanisms forming planets and the timescales of planetary migration and atmospheric evaporation remain to be probed observationally. To shed light on these mechanisms, we performed a combined photometric and spectroscopic analysis of several young transiting systems at different ages: K2-33 (5-10 Myr), AU Mic (~20 Myr), HD63433 (~400 Myr) and TOI-1801 (600-800 Myr). We characterised the stellar activity and physical properties (radius, mass, and density) of these planets through joint transit and radial velocity fits with Gaussian processes.

TOI-1801 b is the oldest planet in this sample, with a radius of 2.1 Re and a mass of 5.7 +/- 1.5 $M_{\oplus}$, orbiting an M0V star every 10.6 days. HD63433 c is an intermediate-aged mini-Neptune (2.7 $R_{\oplus}$) with an orbital period of 20.5 days. In our analysis, we derived a planet mass of 15.5 +/- 3.9 $M_{\oplus}$. AU Mic b and c orbit an M0 star every 8.5 and 18.9 days, respectively. They have radii of 5.0 and 3.2 $R_{\oplus}$, and masses of 8.7 +/- 2.5 and 12.9 +/- 3.2 $M_{\oplus}$, respectively. Finally, K2-33 b is the youngest transiting planet discovered to date with a radius of 5.0 $R_{\oplus}$ and a 3σ upper mass limit of 17.5 $M_{\oplus}$. Our results indicate that very young and close planets as AU Mic b or K2-33 b appear to have a larger radius for their corresponding masses than their older counterparts, while older (TOI-1801 b, HD63433 c) or longer orbital period (AU Mic c) planets resemble older planets. Since these short-period planets are affected by the radiation from their young hosts, they likely lose part of their atmospheres. If indeed the case, our study implies that the mass loss occur on short timescales, favouring the photo-evaporation mechanism.