So, if I craft a level 3 Leather Boots, when I get level 4 it will level up also??
No, but when you make it it's always at your level. So if you can make multiple copies of the same recipe, you can plan multiple crafted upgrades over the course of your game. With some of the recipes (ex: Large Steel Bar = two-handed axe), the ingredients are so plentiful that you'll have no problem crafting an upgrade every single level; with something like Sword of Planets it won't quite be that often, but you can make one as soon as you get the recipe together, another halfway between that and endgame, and a third one when you're level 19 or 20 (assuming average luck; with bad luck you might only be able to make the sword twice).
