As mentioned by DracoSuave, the key restriction imposed by magic item levels is that you generally cannot create magic items that are higher level than you.
They also serve as guidelines for when you might reasonably be expected to find the items. For example, a DM who uses the core treasure parcel system system would give out a single magic item 4 levels higher than you once per level. Of course, a DM is free to ignore these guidelines if he wishes.
Otherwise, there is nothing to prevent a 1st-level character from using a 30th-level magic item.