In 1996, when he was 20 years old, Alex Rodriguez hit .358 with 36 home runs and 59 walks, played Gold Glove defense, and scored 141 runs in 146 games. It’s a performance he’s never improved upon.

That’s no criticism of Rodriguez; he’s had seven more years in which he’s been just as good, which is why he’d be a first ballot Hall of Famer if he retired tomorrow. It’s just a powerful example of the problem with expecting a young star to age the same way the average player does: They often can’t get better than they already are. If a player hits .358 at age 20, it doesn’t follow that he’s going to hit .430 when he’s 27.

A player is a young star precisely because he’s achieved his potential early. The very fact that he is not an average player means he’s probably not going to age like one – improving throughout his 20s, reaching a peak around 27, sustaining that performance for a few years, and then rapidly declining. Instead, great players commonly hit a peak early and sustain it past the age when lesser players have already fallen out of baseball, with a few spikes along the way.

All this brings us around to David Wright, who’s been the best player in the National League this year, hitting .336 AVG/.402 OBA/.606 SLG with 18 home runs, 11 stolen bases, and spectacular (if erratic) defense while playing every game. If you’d told me this March that he’d hit like this, I simply wouldn’t have believed you. His improvement this year is as dramatic and even more unlikely than Jose Reyes’s.

After last season, few players in the majors were less likely to drastically improve than Wright. Not only had he performed superbly at a young age – at 22, he hit .306/.388/.523 – but he did it with a more or less fully formed body and a fully formed game. Reyes held his own in the majors last year despite being absurdly thin by today’s standards and having little idea how pitchers were working him; it was easy to see that a bit more physical maturity and understanding of how pitchers were working him would pay dividends.

Wright, by contrast, was physically mature and had exceptionally advanced pitch recognition, discipline, and ability to hit the other way – put another way, there weren’t any obvious areas where he could substantially improve. One could expect him to gain power with a bit more filling out and understanding of when to turn on the pitch, but one could also expect that this would be offset by a slight loss in average, as reflexes and speed are in decline virtually from the time a player first makes the majors. A bit more power, a bit less average, and steadier defense – that’s what you could reasonably have expected from Wright this season, whether you were coming at the question from a strictly observational point of view or projecting based on his past statistical comparables.

Instead, he’s radically improved. The huge increase in his average has somewhat overshadowed the even more amazing increase in his power. Last season, his isolated power (slugging percentage minus batting average, a good proxy for raw power) was .217 – an excellent number, especially for a 22-year-old, comparable to what Chipper Jones has done in his career. This season it’s .271,which is Vladimir Guerrero territory. Last year’s line-drive outs have become singles, last year’s singles up the middle have become this year’s doubles, and last year’s doubles to the opposite field and down the line have become home runs.

It’s a broad-based improvement in all areas of hitting, and my best guess is that it’s real and sustainable, based on something you can see when watching him – he just has more power and a quicker stroke than he did last year. He’s the same player, just better – something that happens pretty rarely for someone who was so good to begin with.

Assuming that Wright hasn’t simply been playing at the top of his range so far this year (and that’s possible), but that he’s developed instead, it has pretty incredible ramifications. Whereas after last year he looked like an excellent bet to have a Hall of Fame-caliber career along the lines of Ron Santo’s, he’s looking right now like a right-handed George Brett. There’s a long, long way between being 23 and retiring with 3,000 hits, but anyone capable of the kind of fundamental improvement Wright has shown so far in his brief career can’t really have any limits put on them.

If there’s one caution here – and I say this having yesterday written without qualification that I think Jose Reyes is from this point on going to be a better player than Derek Jeter, and someone who similarly thinks that Wright is going to be more valuable than Alex Rodriguez going forward – it’s that the Mets have gotten very lucky. You can have two players with this kind of talent, pay every attention to their development, and have them work both hard and smart, and still not be so fortunate as to see them both hit their upside. In Reyes and Wright the Mets have even more than we all thought they did a couple of years ago; that doesn’t mean Lastings Milledge is going to grow up to be Gary Sheffield and Mike Pelfrey will be Kevin Brown. On the other hand, who knows? It wouldn’t be any less likely.

tmarchman@nysun.com