In his 70s and still appealing to a diverse crowd, Willie Nelson filled the Shell Sunday night, serenading almost 6,000 fans.

Willie Nelson, the 74-year-old Red Headed Stranger and country superstar, draws a wide-ranging crowd to his concerts. Twenty-somethings and younger fans respect that Nelson is older but can still sing, and with gusto; Boomers jive with his carefree demeanor and trademark braids and bandana; others admire his activist pursuits, such as the promotion of biodiesel fuel and the relaxation of federal marijuana laws. One thing unites these cohorts of Nelson fans: the cowboy hat, which was seen in every possible incarnation, color and shape at Nelson’s performance at the Constellation Brands-Marvin Sands Performing Arts Center on Sunday night.

With a “howdy,” bluegrass mandolinist Ricky Skaggs opened the concert for Nelson, priming the audience by asking, “Y’all ready for a little bluegrass music?”

Skaggs shared the stage with Kentucky Thunder, a band of six bluegrass virtuosos playing guitars, banjo, fiddle and double bass. Their 15-song set included ballads that emphasized Skaggs’ vocals as well as fast-picking, fiddle-intensive instrumentals like “Bluegrass Breakdown.”

Skaggs spoke about the importance of respecting one’s musical roots, and he honored the bluegrass greats Bill Monroe, Earl Scruggs and Lester Flatt.

“So many young bluegrass groups don’t go all the way back, but you can’t know where you’re going if you don’t know where you’ve been,” said Skaggs. The band played recent songs as well, such as the 2004 surprise mainstream hit “Simple Life,” which had garnered substantial airplay on country radio stations.

“They were playing a bluegrass tune, and they didn’t even know it,” said Skaggs. A gospel tune, “The Family Who Prays,” rounded out the set.

As fan Jean Tuck of Penn Yan pointed out, “a lot of times, the opening act is just as good as the main attraction.”

The stage lit up again with Nelson and backup musicians playing against the backdrop of a Texas flag, opening with hits “Whiskey River,” “Still Is Still Moving to Me” and “Beer For My Horses.” Some favorites, like “Crazy,” came out early in the set; others, like “Georgia On My Mind,” “Mamas Don’t Let Your Babies Grow Up To Be Cowboys,” “On the Road Again” and “Always On My Mind” were scattered throughout the show. Like Skaggs, Nelson offered a gospel tune, the hymn “I’ll Fly Away.”

The audience — between 5,800 and 6,000 fans were in attendance, crowding the Shell almost to capacity and filling the lawn under a nearly-full moon — didn’t just clap, it yahoo’ed and yee-hawed as Nelson pumped his guitar-strumming arm in the air in time with the music.

Some, like Dev Nagel of York, Livingston County, were at their first Nelson concert; others, like Willie Nelson look-alike Pete Lang of Mendon and his fellow frequent concert-goers Sheila and Havah Storms of Ontario, Canada, had seen 20 to 30 Nelson concerts apiece.

Gary Green of Canandaigua knew exactly why Nelson’s appeal endures: “He’s old, and he still rocks,” said the fan.