If you'll recall, in the original ending of Chrono Trigger, when the party members are leaving, Magus goes through the portal without a word. Lucca says, "So, you're going to look for Schala?" He doesn't answer, but she's right. As you'll know if you've played through Chrono Cross, when Lavos was summoned in the Ocean Palace, Crono was killed, (though saved when the party replaced him with a dummy if you did the sidequest,) Zeal was sent to the Black Omen, Janus (Magus) was sent to the Middle Ages, and the three Gurus were sent to their respective periods. However, Schala, the one closest to Lavos, was fused with Lavos and the formation was sent to the Darkness Beyond Time. In order to save herself, she created a clone of herself and sent it back to the Present, where it was found and raised by Lucca. That's Kid. Magus somehow figures this out, and goes through time (it's unknown exactly how, as the gates are closed by this point and Crono has the Epoch, assuming it wasn't destroyed during the game,) to save her when Lynx attacks Lucca's home and kills her along with all of the other children sans Kid thanks to Magus, who is now passing himself off as Magil. Radical Dreamers takes place about ten years after these events. Thus, as stated in the opening to the game, Magil is probably in his early thirties or so.