Minecraft wiki entry for Before_14w02a, associated text: If your current level is 14 or below, you need a static 17 experience to gain the next level.
For current levels from 15 to 29, each level takes 3 more experience, amounting to (17 + (currentLevel - 15) * 3) experience to gain another level.
At and above level 30, each new level takes 7 more experience, totaling (62 + (currentLevel - 30) * 7).