Mendelssohn is an example of a child prodigy who never lived up to his early promise - with the exception of the Violin Concerto. The Octet and the MSND Overture are the best things he ever wrote.
Oh and btw very little of the music that Mozart wrote as a child is all that highly regarded - apart from the curiosity of it having come from a child. He was actually a late bloomer as a composer and only started writing consistently great music in his mid 20s.
Your knowledge of musical composition is on a level with your knowledge of piano playing.
There may be the need to make a distinction between being a child performing prodigy, and being a compositional prodigy.
Agreed that his earliest stuff is merely ‘good’ to ‘very good’, but we don’t know how good of a performer he was at that age, relative to the standards of the time.
Many of the greatest musicians in history would be interested to learn that Mendelssohn's 'Italian' Symphony, his Trio in D minor for piano and strings, his Octet, his Variations Seriouses, his Songs Without Words, his On Wings of Song, his Rondo Capriccioso etc
are not worthy of music 'expert' Borges tastes.