Four-man outfields. High-tech anti-spying rules. A starting pitcher facing just one batter in a playoff game.

But beyond all the shifts, analytics and social media outreach, here’s the best way to tell Major League Baseball has zoomed into a new era: There’s not a single active player left from the 20th century.

Not one.

Adrian Beltre and Bartolo Colon were the last, the Elias Sports Bureau said. And with all 30 teams set to play Thursday — from Bryce Harper’s home debut at Citizens Bank Park to Mookie Betts and the champion Boston Red Sox visiting Seattle — this year MLB becomes the first of the four major sports without someone still around who played in the 1900s.

The last time that was true in the big leagues? Back before even the World Series existed.

Already this season, the great Ichiro Suzuki has retired, done at 45 after two hitless games last week as the Seattle Mariners swept Oakland at the Toyko Dome.

“I really wanted to play until I was 50, but I couldn’t do it,” he said.

Yankees lefty CC Sabathia says it’s his last year, and so does Giants manager Bruce Bochy.

But, as always, youth springs eternal. Vladimir Guerrero Jr., Fernando Tatis Jr. and Eloy Jimenez lead a strong rookie crop of boppers who might stick around for a while.

Will they be the next Mike Trout or Joe Mauer, or the future Manny Machado or Chase Utley? We’ll see over the years.

In the meantime, after a long winter of waiting, it’s time for everyone to play ball.

“Opening day, since I was a kid, I feel like it’s a celebration of our sport, so it’s something I always look forward to,” Yankees manager Aaron Boone said.

A look at the lineup going into Thursday:

Jacob deGrom, fresh off a big contract boost from the New York Mets, starts against Washington ace Max Scherzer at Nationals Park. DeGrom earned his first Cy Young Award last season, and three-time winner Scherzer finished second. It’s a similar duel at Tropicana Field — AL Cy Young winner Blake Snell of the Rays faces runner-up Justin Verlander and the Astros.

Harper takes his first swings since signing a $330 million deal with the Phillies (that was a record until Trout got $426.5 million from the Angels soon after). Harper has five careetr home runs on opening day, tied with Albert Pujols and Ian Kinsler for most among active players, and will take on Julio Teheran and the NL East champion Braves. Harper is 18 for 50 (.450) with a personal-high eight homers against the Atlanta right-hander.

Machado starts up with San Diego, Paul Goldschmidt is with St. Louis, Robinson Cano got traded to the Mets, Nelson Cruz swings for Minnesota and the excitable Yasiel Puig is in Cincinnati.

Josh Donaldson joined the Braves and is expected to be in action this weekend.