What a place to find yourself at 15 years of age. Your first international swimming race and you are in London, at the Olympics, in a final, in a swimming arena that is deafening because of the local support for Rebecca Adlington, the world champion, the gold medallist at 800m freestyle in Beijing four years earlier and hotly fancied to repeat that feat on home ground.

The race for gold was to be a duel between Adlington and Denmark’s Lotte Friis. Then the gun goes and you change everything.

That night, August 3rd, was the first Olympic statement of intent by American Katie Ledecky and the beginning of a new era in swimming. Over the next two weeks, Ledecky will probably stand alongside gymnast Simone Biles and Usain Bolt in making the Rio games her own. What Ledecky did that night prophesied an Olympic cycle of utter domination.

Swimming prodigies

There has always been an element of Hollywood child star about Olympic swimming prodigies.

Michael Phelps

, who has loomed over swimming much as

Christ the Redeemer

looms over Rio, was also just 15 when he debuted at the Sydney Games in 2000. But in London, Ledecky was notionally there as a support act in the usual US pageant of swimming superstars, the youngest of the von Trapps, as it were.

In 2012, the USA sent a strong team led by Phelps, Ryan Lochte and Missy Franklin. And even though they all delivered, Ledecky's achievement in that 800m freestyle race was the one which caused jaws to drop. She had sliced some five seconds off her personal best that July just to make the Olympic team and reduced that by a further five seconds in the final that night.

At her first press conference as Olympic medallist, she was asked a question about doping allegations, which she dismissed with a combination of candour and poise. Since then, the doping questions have died away but Ledecky has continued to erase and rewrite world record logs at a furious rate. In that 800m event, for instance, she now owns the first 11 fastest times in history. She has broken 11 world records.

At the US trials in Omaha, she finished first in the 200m, 400m and 800m freestyle events. So in Rio, she has the opportunity to emulate Debbie Meyer’s feat at Mexico in 1968 in swimming to gold in those three events. As Ledecky continues to dominate, she has stepped forward as the lone superpower of US swimming. The sheer longevity of Phelps in a may-fly sport is a wonder but there is a sense that at Rio he will be a grandee of the pool, reminiscent of an era already passed. Neither Franklin nor Lochte is expected to take gold in their individual events here.

Competitive instinct

The striking thing about Ledecky is that she is still on the cusp of adult life, with enrolment at Stanford due in September.

Ledecky comes from a relatively privileged background: even her parents have confessed surprise at her ferocious competitive instinct. The reputation of so many Olympic sports has been shattered by cheating that any demonstration of brilliance that seems to breach what is believed possible requires a willing suspension of disbelief. But even in the hard-bitten world of swimming, nobody is questioning that Ledecky is for real: they are simply at a loss as to how to fully explain her persistent, radical improvement. Katie Ledecky swims in the 400m freestyle heat on Sunday at 6.51pm