Nico Muhly, composer of the opera , performed some of the (the creepily haunting No5 and harder-driving No6 the night I saw the performance). He also was the musician "in charge" at the historic performance of at BAM earlier this autumn, when Glass and Steve Reich played on the same stage for the first time in 30 years after an infamous falling-out. Muhly was once an intern in Glass' studio, later moving up to being the person to program Glass' handwritten scores into a computer.

Muhly says working for Glass has taught him "what it means to be a responsible member to the community of musicians" - Glass is known for being a generous composer and collaborator when it comes to the business of making art - and that Glass has "a spirit of benevolence which is rare in composers". The represent a "big overview of his harmonic language", Muhly says.

"When I was working for him you could see things from the form the germ of an idea," he says. "The fun of an is that it's a study. It's designed for technical or harmonic obsessions."

All of them, Muhly says, are "little diagrams of where his head is at the time".

Seeing the progression of a couple of decades of those diagrams compressed into one sitting was incredibly satisfying. In the same way I feel happy to be a reader while Toni Morrison is writing, I have felt incredibly lucky to be alive while Glass - to me, arguably the most important American modern classical composer - is working. It's great to see the work as it comes out whether I love it or hate it. Ever since I talked to him about mortality - Glass doesn't "have retirement in mind in any way at all" - I often wonder each time I see him play whether it could be for the last time. He does carry a performance schedule that puts musicians less than half his age to shame. But when I saw Glass perform at the 2,000-year-old Temple of Dendur exhibit in New York's Metropolitan Museum of Art a couple of years ago, I had the distinct sense of the fleetingness of time, and a sense that, while his music will last, the man himself (like all of us) has a finite number of days on this earth.

drive this home: and , played by Glass both nights, are relatively gentle, like lullabies. (Indeed, I have listened to and often as I've fallen asleep.) The charge and drive largely get more aggressive and passionate as the numbers increase into adulthood, conjuring comparisons to George Gershwin and Aaron Copland. It is hard to imagine falling asleep during , played and ended with a particular flair by Timo Andres. But by (Maki Namekawa, who totally became one with the music), the have slowed down to a Schubertian pace, ending so gently that silence and sound nearly blend into one another - much the way old age can render the elderly as calm as a baby again, as life and death overlap.

As satisfying as seeing the work was the ethnic and musical diversity of the musicians. I talked to Glass in 2012 about the lack of racial diversity in "art music", growing up in his father's record stores in Baltimore ("The city may have been segregated, but my taste in music wasn't") and how he has dealt with race in his work. Diversity isn't unusual with Glass (as in his opera , the Helga Davis lead role in , and the inclusion of bands such as Das Racist at his annual Tibet House fundraiser).

Still, these are exceptions, and so it was great to see the solo piano of one of our most important living composers interpreted not just by white male performers but by Namekawa as well as Aaron Diehl (a jazz pianist who has toured with Wynton Marsalis), Tania Leon, Sally Whitwell and Jenny Lin.

The two most disappointing things about the performance were the venue and the performance by the composer himself. BAM is a cultural institution which is unparalleled in quality not just in New York City but in the entire US. Yet its Howard Gilman Opera House was not an ideal setting for a solo piano concert of Glass' work.

Even sitting relatively closely, I found myself straining to hear the piano in the cavernous space, distracted by many other shifting listeners and the sound of sirens from outside, probably coming from the NYPD following the Eric Garner protesters. (Not that Glass would have minded this juxtaposition of art and street protest: he famously gave a "mic check" to Occupy Wall Street protesters gathered outside his opera at Lincoln Centre in 2011.) I could only imagine how much more moving it would have been to hear these works in Carnegie Hall, the kind of venue whose acoustics illuminated the subtlety of the second movement of in its American premiere in 2012, or Glass at the keys himself with violinist Tim Fain in .

Regarding Glass playing , I should qualify my disappointment at hearing my favourite composer play perhaps my favourite piece of music. Glass' technique did seem to be softening, and he didn't seem nearly as sharp as when I last saw him play , in a programme with Patti Smith a couple of years ago; nor did he seem to have the boundless energy with which he kicked serious ass in playing , for hours on end, that same weekend.

But there is something sweet, if melancholy, in seeing Glass slow down a little as he approaches 78. A choreographer can't dance their own work forever, but that doesn't mean it's any less important. No matter how much I may admire Glass as a performer, his legacy will be primarily as a composer. Indeed, he composes precisely so that Muhly, Diehl and others not yet born can perform his work differently than he ever could.

And so, his work will outlast not just his hands and the hands of the pianists playing them this month, but my own ears, too - and yours.

It works best, therefore, to think about the as akin to the Metropolitan Museum's 2003 show on Leonardo Da Vinci's notebooks: as the sketches of a creative force whose work will outlast any one of us, including the artist who made it.

The 20 will be performed in full again at the Barbican Centre in London in April 2015.