Brandon Belt turned 30 on Friday, joining the majority of his Giants teammates, 16 of whom are in their 30s.

As we’ve been told when Buster Posey, Brandon Crawford and other Giants reached this age, 30 is just a number. It doesn’t mean an immediate statistical demise or sudden loss of eyesight. Though some Giants fans might think that already has transpired for Belt.

Au contraire.

Three weeks into the season, the Giants’ first baseman has been an easy target for frustrated fans of a team that was last in the majors in runs entering Friday and began the day at 7-11 after losing five of six.

They’ve seen him strike out looking to end a 10-inning game and a few other times and, judging from social media rants and barstool chatter, figure all he wants to do is walk.

Even if it means taking close pitches in two-strike counts.

But a dive into the statistics, as small a sample size as it is, suggests Belt is getting a bum rap.

“I think I’ve shown over my career I have a pretty good strike-zone awareness,” Belt said. “I know where the strike zone is. For me to be successful, I can’t swing at balls. I’ve got to swing at strikes. That’s just the way it is.

“Hopefully, the umpires and I are on the same page that day. As far as me being successful at the plate and driving the ball and having the best chance to get hits, that’s me staying in the zone.”

The numbers entering Friday, courtesy of the good folks at Baseball-Reference.com and FanGraphs, contradict some people’s perception of Belt.

As to whether Belt has struck out looking too many times this year … of his 18 K’s, five were looking, and 59 batters have taken more third strikes, including Jose Altuve, Paul Goldschmidt and J.D. Martinez.

As to whether Belt should be more of a free swinger … he swings at 26.1 percent of the pitches he sees, and only 14 batters swing at a higher clip, including Bryce Harper, Giancarlo Stanton and Joey Gallo.

As to whether Belt strikes out too much … 53 batters have struck out more, including teammate Hunter Pence, and Evan Longoria and Crawford have just as many K’s.

As to whether Belt exercises too much patience … he swings at first pitches 50.8 percent of the time, and only Toronto’s Aledmys Diaz, whose on-base percentage is nearly 100 points lower, swings at first pitches more often.

There’s actually a good reason Belt shouldn’t be too patient at the plate, and that’s his two-strike batting average of .167 (5-for-30) this season, which is on par with his .163 career mark.

But swinging at pitches out of the zone is not how Belt developed into a keen-eyed hitter with a high on-base percentage and $72.8 million contract.

“When I go outside the zone, I’m way worse than if I stay inside the zone,” Belt said. “We’re facing big-league pitchers here. This is not high school where they’re throwing 75 (mph). These guys are throwing 95 with movement.

“You can’t sit there and say, ‘Oh, the ball’s barely at the plate, I’ve got to swing here.’ It’s a reaction. My reaction is to swing at strikes and lay off balls. That’s just all there is to it.”

One reason Belt doesn’t go out of the strike zone, he said, is he’s not good at it. Indeed, his contact percentage on pitches he swings at out of the zone is the second-worst in the majors — 38.1 percent, 185th out of 186 qualifying hitters — better than only Kansas City’s Jorge Soler.

On the other hand, Belt’s career contact rate out of the zone is 60 percent.

(Curiously, Cincinnati’s Joey Votto, known for rarely expanding the zone, just 16 percent of the time, fourth-lowest figure in the majors, actually has the second-highest out-of-zone contact percentage, 90.5.)

So Belt mostly lets the bad pitches go, though the percentage of times he swings out of the zone, 29.4, is higher than that of Giants teammates Joe Panik and Andrew McCutchen.

There are exceptions to the rule. Sunday in San Diego, the Giants scored just once, on Belt’s sacrifice fly, coming with one out in the second inning. Twice in the plate appearance, he swung at pitches out of the zone, the second time resulting in a ball driven deep enough to score Panik.

“There are definitely times when I do want to expand a little bit,” Belt said, “like I did (Sunday) with a guy on third and less than two outs, you want to get that ball in play so we can score that run. But other than that, I know which balls I can hit well, I know which balls I can’t.”

In Arizona, Belt homered both Wednesday and Thursday, off closer Brad Boxberger’s 3-2 changeup, the game-winner in the 10th inning, and Zack Greinke’s 2-2 slider, coming in Thursday’s 3-1 loss.

Belt swung out of the zone once in the at-bats’ 12 pitches, and that was on a first pitch (foul ball) with a runner in motion. He didn’t strike out looking in the series and got the Giants’ only hit in Tuesday’s opener, on an eighth-inning check swing.

A knock on Belt is that when he does strike out looking, it tends to come in high-leverage situations, especially April 8 when his strikeout ended a 10-inning, 2-1 loss to the Dodgers. Belt pinch-hit with a man on second, and his six-pitch at-bat ended when he watched Kenley Jansen’s 91 mph cutter that both Belt and manager Bruce Bochy swore was high.

Making it worse in fans’ eyes was that it came one day after McCutchen’s 14th-inning walk-off homer that capped a 12-pitch at-bat in which he fouled off seven pitches.

Belt struck out looking three straight games (April 7, 8, 9), and all were considered borderline pitches, according to MLB Gameday. On April 9, in the ninth inning of a 2-1 loss, he took strike three, a low pitch that could have been called a ball.

Ditto on a high pitch in the eighth inning of the McCutchen game. The next batter, Crawford, struck out looking on a pitch way outside.

More than once following a called third strike, the usually tame Belt got fired up and had words for the umpire.

“I don’t want to strike out,” he said. “I want to hit the ball and get on base, give my teammates a chance to drive me in. I think in those situations, that was when somebody was in scoring position, and we needed those runs. You don’t want the bat taken out of your hand at that point.

“Those are the ones that are most upsetting, but they’re all upsetting. In that case, you know you’re not getting fooled by the pitcher. You see the ball. It’s a ball the whole way, and they call a strike. Well, what are you supposed to do?”

John Shea is The San Francisco Chronicle’s national baseball writer. Email: jshea@sfchronicle.com Twitter: @JohnSheaHey