Luther's Catechism has a wonderful section on this - and how an infact/child cannot examine theirself (easier than saying himself/herself...ha). The Church's doctrine must be followed by those partaking in Communion, and although they may be saved/believers, they must be examined before partaking.
Churches typically have an age for examination. Of course this age is somewhat arbitrary - but to keep it open, one year an 11 year old will say they are ready..and maybe they are! But once ONE parent sees that then lil' Suzie too is ready at 11, and so is Johnny. Then next year Bobby will be ready at 10 years of age...eventually this falls apart. Hence the arbitrary age for Confirmands that is appropriately set.

Grace is not common.