How many of the great composers wrote their first piano (or other) concertos at or before age 18?

J.S. Bach began composing as a teenager. His son, Carl Philipp Emmanuel Bach, wrote his first compositions at the age of 16. Beethoven was a child prodigy, composing very early as well.

“Liszt, Saint-Saëns, Franck, Handel, Beethoven, Mendelssohn and Richard Strauss all composed at very young ages. Many lesser figures did too.” Source

Chopin wrote his first piano concerto at 20 (though this is older than the age limit given here).
Shostakovich wrote his first symphony at age 20 as well.
Saint-Saëns wrote his first piano concerto at age 16.
This guy has written quite a few orchestral works at his young age.
There are many more examples out there that I just haven’t bothered to list.

I know Beethoven wrote a couple of pieces for piano before age 12.

Mozart wrote quite a few.

Childhood symphonies (1764–1771)
These are the numbered symphonies from Mozart’s early childhood.
Symphony No. 1 in E-flat major, K. 16
Symphony No. 2 in B-flat major, K. 17 (spurious, attributed to Leopold Mozart)
Symphony No. 3 in E-flat major, K. 18 (spurious, by Carl Friedrich Abel)
Symphony No. 4 in D major, K. 19
Symphony No. 5 in B-flat major, K. 22
Symphony No. 6 in F major, K. 43
Symphony No. 7 in D major, K. 45
Symphony No. 8 in D major, K. 48
Symphony No. 9 in C major, K. 73/75a
Symphony No. 10 in G major, K. 74
Symphony No. 11 in D major, K. 84/73q
Symphony No. 12 in G major, K. 110/75b
Symphony No. 13 in F major, K. 112

Edited: Sorry I didn’t see you hadd listed Mozart in the question. It would be intersting to go down the list of childhood symphonies and listen to each one. Seeds of greatness.

I don’t know, but I do know that Mozart learned piano by watching his older sister practice when he was only two.