Marlon Stewart remembers the first time it happened.

“When I was at Cal, there was one time I was in Las Vegas for the summer recruiting time,” the first-year Hawaii basketball assistant recalled. “I got into a cab and the (cabbie) said, ‘Sir, you have to be 18 to ride in a cab by yourself.’ I think I was like 24 or 25 years old. At that point it kind of started.”

Stewart, 32, has heard it all. The staffer who showers post-practice in the fountain of youth. The guy who is definitely, positively younger than senior Gibson Johnson, and maybe half the roster. Stewart has (for the most part) worn his faux boyish distinction with a smile since well before his arrival to UH this summer.

“It never fails,” he said.

It’s true, the Mercer Island, Wash., native looks like he could suit up and play for the Rainbow Warriors, instead of coordinating their defense and guiding post play. But his appearance belies a steady, no-nonsense tone in practices and on the bench that has helped the ’Bows to a 3-0 start.

Stewart will wring his defensive mind — honed at rungs on the coaching ladder like Washington State, Cal, Virginia and Montana — for the means to slow down Friday’s opponent, Nevada (3-0). UH’s old WAC foe is prowling around the edges of the Top 25.

He was hired to replace Chris Acker, who took an assistant job at Boise State in the offseason after joining head coach Eran Ganot the past two years in Manoa. Stewart got his breakthrough last season as a full assistant in Missoula, Mont., promoted from director of operations by mentor Travis DeCuire. He already had credentials: someone who’d been to two NCAA Sweet 16s, and until the 2016-17 season, a member of 10 straight postseason teams.

He’s obsessive about nuances on D, which gradually became his forte after getting his “in” to the business as the rare video guy who could transition seamlessly among VHSs, DVDs and computers in the early- to mid-2000s. He specially admired the compact-style Pack Line Defense espoused by Dick and Tony Bennett while he first toiled as a tape-splicing student at WSU.

“I started playing point guard in high school, but I was a guard that couldn’t dribble, couldn’t shoot,” Stewart said. “So defense was my ticket to even just play. Defense in pretty much every facet has intrigued me.”

So far, so good. UH’s opponents in the season-opening Outrigger Resorts Rainbow Classic were held to a combined 40.1 percent shooting from the field, including 34.9 percent by Troy in a narrow overtime win for the ’Bows.

Stewart actually hasn’t changed a whole lot of UH’s defense to the naked eye; even subtle tweaks require a give and take with Ganot and the rest of the established staff.

This week Stewart proudly noted a couple of hard-earned gray hairs that he intends to keep.

Ganot is in on the youth jokes — “he looks like he’s 5” — but in the same breath speaks to Stewart’s thoroughness, attention to detail and desire to help players improve.

“I don’t know if he can do much about how old he looks,” Ganot said. “I mean, I don’t even know if he can grow a goatee or a beard or any of that stuff. But he can certainly gain the respect of these guys, and he has, because of what he’s about.”

Johnson, the second- oldest active college hoops player in the country at nearly 26, is the recipient of all the “old” jokes, so in that disparity he and his position coach have bonded.

“I think the age difference between me and Justin (freshman Hemsley) is the same between me and Coach Marlon,” Johnson said with a smile. “So, that’s pretty funny. Just how it is with me, how I’m older and playing basketball, I don’t really care because I’m here and I’m playing. So for him to be young and in the position that he is, that only speaks to how good as a coach that he is and what he knows. To get to the level he’s at at the age that he’s at, it only happens to people who’ve done the work to deserve it.”

Stewart put in the time in another important arena. He proposed to his girlfriend, Morgan, a day after getting the Hawaii job. She said yes.

“When we moved we were looking at pictures,” Stewart said. “There was a picture of me from high school, and my fiancee looked and said, ‘You literally look the exact same.’ ”