Build a Solid Foundation
Algebra was invented by Arabs but sounds like Greek to kids who are not clear on the concepts under the equations. Its not that kids shouldn't be taught algebra - its just that its necessary to make sure they are building on a solid foundation and 'owning' each concept as they move ahead. Likewise, while a motivated kid might read Twilight or Harry Potter before a passing a fascinating standardized test at the same reading level, it is also important not to discourage new readers by presenting them with work at frustration level.
Fractions can be taught to five-year olds using manipulatives and pizza analogies, so this principle is not saying to hold children back. However, giving a junior high student a sheet of work doing decimal division when they don't have a solid grasp of exactly what decimal places mean is asking for trouble.
Finding out what your child understands and what they don't, then giving them work that builds on what they understand is one of the keys to effective teaching.