Age 30 is an important dividing line for a professional baseball player.

Obviously, players can have a long and prosperous career after turning 30, but any signs of decline become more concerning once someone has crossed that threshold, even if it's one of the best players in the game.

The seven star 30-somethings below didn't perform up to their usual standard at the plate in 2023, which makes the upcoming season a pivotal one for all of them as they look to prove last year was an aberration and not the beginning of a decline phase.

Nolan Arenado, 3B, Cardinals
2022: 150 wRC+ | 2023: 107

St. Louis’ pitching staff shouldered much of the blame for the team’s last-place finish in 2023, but Arenado certainly didn’t help matters with his slow start and poor finish. The veteran third baseman had a .620 OPS through May 10, contributing to the Cardinals’ 13-25 record over their first 38 games. Although Arenado eventually got back on track and concluded the first half with strong numbers, a back injury led to a second-half slide and a premature end to his season in September. Excluding the shortened 2020 campaign, last year marked the first time Arenado didn’t record 30 homers or 100 RBIs since 2014. His OPS also dropped 117 points from 2022 (.891), when he finished third in the National League MVP race, to '23 (.774). If the Cardinals are going to return to postseason contention in 2024, they’ll surely need a more consistent season from Arenado, who will turn 33 in April.

Paul Goldschmidt, 1B, Cardinals
2022: 176 wRC+ | 2023: 122

The Cardinals’ other star hitter also took a step back last season, one year after winning the NL MVP Award with a .317/.404/.578 slash, 35 homers, 41 doubles and 115 RBIs over 151 games in his age-34 campaign. Across 154 games in 2023, Goldschmidt slashed .268/.363/.447 with 25 homers, 31 doubles and 80 RBIs, notching his lowest OPS (.810) since '11. On the plus side, many of the first baseman’s underlying metrics -- including his hard-hit rate, barrel rate, walk rate and expected wOBA -- either held steady or improved from his MVP season, which bodes well for the 36-year-old entering 2024.

Teoscar Hernández, OF, Dodgers
2022: 130 wRC+ | 2023: 105

While Hernández still had 26 homers and 93 RBIs during his lone year with the Mariners, he went from posting an .852 OPS for the Blue Jays across 2020-22 to just a .740 OPS last season. Exchanging pitcher-friendly T-Mobile Park, where he had a .643 OPS in 2023, for Dodger Stadium should help the 31-year-old’s efforts to bounce back in '24, though he may also need to curtail his elevated strikeout rate (31.1% in '23) to return to his previous heights.

Manny Machado, 3B, Padres
2022: 153 wRC+ | 2023: 114

Coming off a standout 2022 season that saw him finish second in the NL MVP Award voting, last year was a tale of two halves for Machado. After the third baseman posted a .678 OPS over the first three months, he managed to bounce back considerably with an .878 OPS over the final three, despite playing through serious right elbow pain for much of the second half. He underwent elbow surgery two days after the regular season ended to address an issue that had bothered him since 2022. Overall, his OPS dropped from .897 in 2022 to .781 last year, but he nonetheless reached the 30-homer plateau for the seventh time in the past eight full seasons (excluding '20). Although Machado may be limited to designated-hitter duty early in 2024, it’s not hard to imagine the 31-year-old regaining his MVP form at the plate now that his elbow problems are a thing of the past.

George Springer, OF, Blue Jays
2022: 133 wRC+ | 2023: 104

After playing only 211 games over his first two seasons with Toronto, Springer stayed healthy last year and appeared in 154 games, the second most in his career. However, the veteran took a significant step backward at the plate. With his hard-hit rate, barrel rate and walk rate all declining, Springer posted a career-low .732 OPS. The Blue Jays have spent the offseason searching for ways to upgrade their offense, but getting rebounds from holdovers such as Springer -- who turned 34 last September and has three years left on his six-year, $150 million contract -- could be just as impactful as any potential additions they can make at this juncture.

Giancarlo Stanton, DH, Yankees
2022: 116 wRC+ | 2023: 89

The Yankees’ decision to trade for Stanton after he won the NL MVP Award with the Marlins in 2017 is an ongoing source of frustration for the club more than six years later. Stanton, whose contract runs through 2027, can still put a charge into the ball when he makes contact, but he doesn’t do it nearly enough to offset his lack of defensive value and injury woes. The slugger reached his nadir at the plate in 2023, slashing .191/.275/.420 over 101 games -- nearly two-thirds of them coming as a DH. Entering his age-34 season, Stanton is facing an uphill climb as he tries to rebound.

Mike Trout, OF, Angels
2022: 176 wRC+ | 2023: 134

Somewhat lost amid Trout’s continued struggles with injuries is the fact that his production dipped last season before he suffered a left hamate bone fracture in early July. Granted, the three-time MVP was still significantly better than league average, but his batting average (.263), on-base percentage (.367) and slugging percentage (.490) were all his lowest since his abbreviated debut season in 2011. With Shohei Ohtani departing in free agency, the Angels are counting on Trout to stay healthy and regain his stature as one of the best players in baseball at the age of 32.