Michael Phelps competes in the 100-meter butterfly at the Indianapolis Grand Prix swimming meet. (Michael Conroy/AP)

A couple decades ago, college-age kids and teens populated swimming. The advent of professionalism and sponsor dollars led to a new era dominated by full-grown, post-graduate men and women.

This season has brought another trend. It’s suddenly very hip to be 30-something – or older — and back in the pool seeking a spot at the 2012 Summer Games in London after several years, or even a decade, of retirement.

The Indianapolis Grand Prix showcased a handful of former retirees preparing for another Olympic run Thursday. There was four-time Olympic medalist Brendan Hansen, 30, who ended a 21 / 2-year retirement with the hope of making his third Olympic team. He finished fourth in the men’s 200-meter breaststroke final Thursday night with a time of 2 minutes 13.93 seconds.

“It’s definitely been a test of patience,” Hansen said. “I told myself I was never going to compare myself to where I used to be. . . . But everybody else does that.”

Swimming in the lane next to him was the top American in the sport before Hansen took over: Lake Braddock High’s Ed Moses, 31, who returned to competition last year after more than six years out of the sport. A two-time medalist at the 2000 Summer Games, Moses faces a considerable challenge to make this year’s Olympic team, but he qualified for the night’s A final and finished seventh overall in 2:16.08.

“It’s going to be tight on time,” said Moses, who attended the University of Virginia. “I knew that when I started. It’s going to be hard. . . . That doesn’t deter me, though, from believing I can do it.”

Another ex-retiree, Anthony Ervin, 30, placed 16th overall in the 100 freestyle (50.85). Ervin, who won a gold in the 50 free at the 2000 Summer Games, took about eight years off before pursuing a comeback last year; he has posted surprisingly fast times in his specialty, which will be contested Friday. Ervin enters with the ninth-fastest qualifying time (22.27)

Ervin, Hansen and Moses represent just a few of the former superstars who ditched their goggles for a few years, but couldn’t stay away. Janet Evans, the one-time teen phenom who retired at the 1996 Summer Games in Atlanta, will compete at the Olympic trials in Omaha in June, a long shot to make her fourth Olympic team at 40 after 15 years out of the sport.

One of the greatest swimmers in history, Australian Ian Thorpe, came out of retirement last year, but his bid to make Australia’s Olympic team failed less than two weeks ago at his nation’s Olympic trials. Libby Trickett, Michael Klim and Geoff Huegill also un-retired to compete.

“When I was growing up, swimming seemed like something you did through college . . . then the rigors of adult life kind of consumed you,” said Ervin, who was in graduate school last year at the University of California when he decided to resume training with the college team. “At the turn of the millennium, there were notable differences in how it’s been professionalized.”

Results here mean little as few of the swimmers are in top form and many, including 2011’s most dominant male swimmer, Ryan Lochte, are smack in the middle of heavy training. Lochte finished ninth in the 100 freestyle in 49.46 seconds as Nathan Adrian (48.62) topped Michael Phelps (48.74) for the gold medal — despite the fact that Adrian’s jammer shorts split in the back seconds before the race.

Later, in the 100 butterfly, Phelps claimed the gold medal in 52.23 and Lochte got ninth in 52.32. In the women’s 400 free, Towson’s Katie Hoff claimed second in 4:07.00 and Kate Ziegler of Great Falls placed sixth in 4:12.98.

One reason Hansen said he didn’t want to compare himself to his old self was that he didn’t want to finish the same way he did in 2008. After winning individual silver and bronze medals at the 2004 Summer Games in the 100 and 200 breast, he finished fourth in the 100 breast in Beijing, his only individual event.

Demoralized and burned out, Hansen got out of the sport fast. He dived into triathlons and helped manage a nutritional supplement company. He had no intention of returning, but missed the competition.

“A lot of the expectations and pressure I had in ’08 was from myself,” Hansen said. “How you react to yourself is really important in how successful you are. Now, I’m going out there with a clear head, a refreshed outlook.”

Moses tried professional golf and a marketing job in Los Angeles. But he never found the same level of success he hit in the swimming pool just a couple years out of high school.

“He’s a competitor,” Hansen said. “He’ll be there racing, no matter what. It’s just that he may run out of time.”

So might they all.