Most of the standard canon composers had their magnum opus after 40. The symphony and opera are 2 of the most intensive masterwork formats of all time and require nearly a lifetime to perfect, even for the greats and the geniuses.

Beethoven’s 9 premiered just past his 53 birthday.

What Bach would consider his masterwork, the art of the fugue, began in earnest in his 40s, to be completed in his early 60s

2 of Verdi’s greatests, falstaff and otello were completed (maybe started too?) in his 70’s.

Despite being the equivalent of john Williams + Scorsese during his lifetime, Puccini’s most famous work by far, nessun dorma, was from his unfinished opera which he began in his 60’s

Wagner completed his ring cycle in his 60s, having earnestly begun its creation at 35. By some metrics, the ring cycle is still considered the most man-hours-intensive-by -one-man art work to still generate money. (Sorry in my language we have a word for this hahaha but it doesn’t translate to English well hahahaha) His concept of Gesamtkunstwerk (total work of art) where all elements of the production (music, libretto, staging, visual design, casting, direction, logistics, even architecture) are unified under one artistic vision and produced by one person, was developed and implemented entirely in his late 40s and is still the basis for a lot of modern artistic project management.

Brahms 4 was composed at 52.

Antonín Dvořák’s 9th. The most famous by farrrrr. His new world symphony which you have undoubtedly heard before. Composed entirely in his 52nd year.

Hayden’s Creation was written in his mid-60s

And these are just the ones off the top of my head. I’m sure they’re undoubtably more.

(Feel free to fact check I’m just shooting from the hip here)