I am currently reading a book by Barry Cooper : Child Composers & Their Works.
The book is a historical survey of child composers before 1900, and a list of their works. He takes to task most past music critics of children's music , as derivative, uninspired, unworthy of consideration, and presents an argument as to why this was wrong. Most of those children who went on to become composers of note, either destroyed, modified or re-wrote these works, and/or, refused to acknowledge them, some from fear of ridicule or association with mediocrity. (the result of savage criticism). Mr Cooper is of the opinion that these works are a valid picture of the composers early development, and few of us will disagree with that.
The youngest composer, for whom he has evidence is Frederick Ousley (1825-89), who wrote piano pieces in November 1828 at the age of 3years & 98 days.They were published at various times in the 19th century.
Mozart was probably the most precocious of child composers, but most of the well known composers have lists of "juvenalia', some of which would be interesting to hear, but some would only interest musicologists.
But this is an interesting book for anyone interested in the musical life of a composers early development.
Mr Cooper is probably well known to most for his completion of the 1st movement of Beethoven's Symphony no10 in E flat . Available on Carlton Classics #3036900042 (includes a talk by MrCooper), & Chandos Chan 6501)
Marcus