The role of stellar age in the measured properties and occurrence rates of exoplanets is not well understood .
This is in part due to a paucity of known young planets and the uncertainties in age-dating for most exoplanet host stars .
Exoplanets with well-constrained ages , particularly those which are young , are useful as benchmarks for studies aiming to constrain the evolutionary timescales relevant for planets .
Such timescales may concern orbital migration , gravitational contraction , or atmospheric photo-evaporation , among other mechanisms .
Here we report the discovery of an adolescent transiting sub-Neptune from K2 photometry of the low-mass star K2-284 .
From multiple age indicators we estimate the age of the star to be 120 Myr , with a 68 % confidence interval of 100–760 Myr .
The size of K2-284 b ( R _ { P } = 2.8 \pm 0.1 R _ { \oplus } ) combined with its youth make it an intriguing case study for photo-evaporation models , which predict enhanced atmospheric mass loss during early evolutionary stages .