On a slight tangent, a friend of mine has two sons. The younger son is around six years old. He used to be quite fidgety during Mass. A month or so ago he expressed an interest in serving at the TLM (it is usually a Missa Cantata). Although I think the rules say that one must be at least the age of reason to serve Mass, an exception was made in his case (circumstances that I won't get into on a public forum). It is amazing how well he has taken to it. Not to mention how adorable he looks in his little cassock and surplice.
The "age of reason" is around seven, on average. I know some kids with more sense than some 30 year olds who may NEVER reach the age of reason . . .
Remember "Little Nellie of Holy God"? She was quite precocious and was allowed by her bishop to receive Communion at age 4, I think.