I think I'd agree with stereolightning that Snape's sequence of memories in DH suggests fifth year -- in fact, before the Snape's Worst Memory scene, which happened during their OWLs. This deeply disappointed me on first reading DH, because I'd always assumed The Prank had made the Marauders grow up quite a bit and that it was their last time really bullying Snape. But if the scene by the lake was after The Prank, then it looks like there wasn't so much growing up done, after all. :/