Which composer's first symphony was referred to as beethoven's 10th symphony, which wasn't composed until he was in his early 40s?

Johannes Brahms (or just Brahms)

Symphony No.1, Op.68 by Johannes Brahms

Explanation:

    When Johannes Brahms (1833 - 1897) introduced his first symphony (Symphony No.1, Op.68), he was 43 years old - a surprising fact for a composer who had always devoted himself exclusively to music.

    Soon after the premiere of the First, the conductor von Bülow named it the Tenth, alluding to the continuity it represented in relation to Beethoven's nine symphonies. However, Brahms retorted with harsh contempt when a Viennese critic noted that the final theme of his symphony resembled that of the Ninth Choir. and the stupid see it faster, ”he said.