We performed a comprehensive study of the stellar populations in the young Large Magellanic Cloud cluster NGC 1866 , combining the analysis of its best-studied Cepheids with that of a very accurate colour-magnitude diagram ( CMD ) obtained from the most recent Hubble Space Telescope photometry . We used a Bayesian method based on new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain ages and initial rotation velocities of five well-studied Cepheids of the cluster . We find that four of the five Cepheids belong to an initially slowly rotating young population ( of 176 \pm 5 Myr ) , while the fifth is significantly older , either 288 \pm 20 Myr for models with high initial rotational velocity ( \omega _ { \mathrm { i } } \sim 0.9 ) , or 202 \pm 5 Myr for slowly rotating models . The complementary analysis of the CMD rules out the latter solution while strongly supporting the presence of two distinct populations of \sim 176 Myr and \sim 288 Myr , respectively . Moreover , the observed multiple main sequences and the turn-offs indicate that the younger population is mainly made of slowly rotating stars , as is the case of the four younger Cepheids , while the older population is made mainly of initially fast rotating stars , as is the case of the fifth Cepheid . Our study reinforces the notion that some young clusters like NGC 1866 harbour multiple populations . This work also hints that the first population , i.e . the older , may inherit the angular momentum from the parent cloud while stars of the second population , i.e . the younger , do not .