This is more a question of story identification, rather than checking on canon. Pre-crisis, Superboy is noted for becoming friends with the Legion's main trio, Cosmic Boy, Chameleon Boy and Saturn Girl, in his mid-teens; they traveled back in time to meet the person who inspired their creation, and thus gave Clark some fellow super-powered teens to pal around with. However, Post-Crisis, Clark didn't begin his super career until well into adulthood.
In the early 2000s Superman series Secret Origin, a retelling of the Superman mythos combining elements from Post-Crisis, Pre-Crisis and whole new takes on older stories, including revamping elements from the golden age [Superboy, Krypto, and the Legion, for instance]. So I'm wondering if there is a story, both Post-Crisis and New 52, which explains when they met, and how the Legion came to be.
Would anyone be able to elaborate?