Ray Allen, right, and Boston teammates Kevin Garnett and Paul Pierce and won’t play in the NBA finals for the third time in four years, having been supplanted by Miami’s fresher and more athletic Big 3 led by LeBron James, left. (Alan Diaz/AP)

One day, Kevin Garnett is a rook, checking into his first game at Madison Square Garden, eyeing an aging Patrick Ewing at the scorer’s table. “What’s up, Gramps?” the 19-year-old rookie said in 1995 to the veteran, who called him “young fella” that night.

The next day, it seems, that kid is 34 — older than Ewing was then, his shot flatter, his legs hardly spry. The fire-breathing competitor in K.G. is still there, but the body of the player who beat his man to the rebound is not. A younger, more agile, maybe hungrier Chris Bosh gobbled up a miss by a teammate and scored. As 39-year-old Shaquille O’Neal watched from the Boston bench.

Miami, four games to one.

Something about these NBA playoffs could make a veteran viewer feel downright decrepit and useless with a remote. Long past our network prime, we are now wheezing in LeBron’s high-def, HDMI cable world. He is not just sneering and dunking on the Celtics; King Fame is dunking on our memories.

If you grew up on Magic and Bird’s NBA like me, and eventually and completely gave in to the majesty of Michael — if you begrudgingly deemed someone after Michael Jordan acceptable to hold the league’s torch aloft — the last few weeks had to put you in touch with your fan mortality.

After the Lakers were swept away in four, I realized Kobe Bryant has now spent two fewer years in the NBA (15) than he did on this planet before he was drafted at just 17.

Memphis, full of young guns, needed just five games to shock a painfully slow San Antonio in the first round, where Tim Duncan, 35, and many creaky-kneed peers look less like future Hall-of-Famers and more like a feeble Willie Mays falling down in the New York Mets outfield at the end of his career.

“Growing old is a helpless hurt,” the Say Hey Kid said that day.

Don’t Kobe, Shaq and Duncan know it. For the first time since 1998, not one of the most accomplished players in the post-Jordan era (led by Kobe’s five, they hold a combined 13 championship rings) will play into June.

What’s more, Garnett, Paul Pierce and Ray Allen won’t be playing in their third NBA Finals in four years, having been supplanted by a fresher and more athletic Big Three in Miami.

This is unsettling because, unlike many other professional sports leagues, NBA consumers are used to established commodities in the championship series. According to the Wall Street Journal, between 1951 and 2010 one of the two NBA finalists had played in the finals the year before 37 times. Even with the expansion of six new teams over the past three decades, the average number of years since one of the two finalists had last advanced to the finals was 2.1 years.

Change throws us for a loop in the NBA, especially when none of this year’s conference finalists — Chicago, Miami, Dallas and Oklahoma City or Memphis, who play Game 7 Sunday, have advanced this far for at least five years. If neither Dallas nor Miami advances to face each other for a finals rematch of 2006, it will be the longest stretch of a finalist not returning since 2000.

Yes, we can probably blame LeBron for this, too.

But really, seeing a worn-down Shaq in street clothes for an elimination game last week in Miami was just depressing.

If you are 40-plus, Shaq was your monster; Kobe was your disciplinarian, who swatted away the wrists and submerged the confidence of younger foes trying to guard him; K.G. was that mouthy veteran who shut the young bucks up till it was their turn.

The tough-to-digest reality for old-school hoophead America today is that it is their turn.

Either LeBron in Miami or Derrick Rose in Chicago, who meet Sunday in Game 1 of the Eastern Conference finals, is eight wins away from becoming an NBA champion. And if it’s not those relative children hoisting the gold-ball trophy, it will be Kevin Durant and Russell Westbrook in Oklahoma City or a bunch of wunderkinds in Memphis, including rookie Greivis Vasquez, who was starring at Maryland, oh, last year.

The only obstacles standing in the way of the new generation are Dirk Nowitzki and Jason Kidd in Dallas — and neither player would have a shot if Jordan the GM hadn’t prematurely traded away Tyson Chandler from Charlotte.

So now that the future is here, there are two ways to look at this development, which isn’t a gradual changing of the guard as much as it’s a Wall Street crash of trusted and true players for whom you used to root. (The Spurs, threatening 70 wins, and the Lakers, 17-1 after the all-star break, were the Nos. 1 and 2 seeds in the West after all, and Boston won three of four from Miami in the regular season. They all got old quick in May.)

One, we can reluctantly embrace Bron-Bron, D. Rose and K.D., to the point where we nauseatingly refer to them by their nicknames. We can do this like we once reluctantly embraced Kobe, Shaq and Tim after Michael, realizing someone had to carry the torch. We can do this in memory of Michael and others, who once had to fight for our allegiance in a post-Magic-and-Larry league. We can understand this is a natural progression for David Stern’s forward-thinking NBA.

Or, two, we can foolishly cling to the past and refuse to change. We can never capitulate to the idea that Kobe is too old and the Lakers, Celtics and Spurs are done as championship contenders. Yes, we can do what I am doing the next few weeks: root like hell for Dallas. Because as the Death Star of youth takes aim, Dirk and J. Kidd are now our only hope.