While many associates the Flying Raijin Jutsu with Minato Namikaze, it was actually Tobirama Senju who created this unfettered ability.
As to how: most likely from Tobirama Senju himself. Conversely, he could have reverse engineered it from information given to him by Hiruzen Sarutobi. They're may also be scrolls with the information needed hidden somewhere in the village.
So he was using it during the third war and I'm assuming he was around 20 during said war.
He never actually learnt the technique, and therefore never performed it. In his Six Paths Sage Mode, his speed is ridiculously high and he can hold his ground even without employing a jutsu like the Flying Thunder God.
The reason why is because he wasn't aware he possessed it at the time. During the Third Great Shinobi War, both Rin and Kakashi were ambushed by mist ninja, they attempted to kidnap Rin.
So both Naruto and Minato were able to evade A's super high speed attack, which means they are really one of the fastest shinobi to ever live. However, Minato had to use Hiraishin (space/time) while Naruto had to physically evade A through the speed he got from the Kyuubi mode.
Is Minato fast without FTG? Yes, shown when A the 4th Raikage tries to charge Minato, Minato has fast enough reflexes to take a kunai and teleport away from A.
Tobirama Senju was the Second Hokage of Konohagakure and likely the man who had created most of the Jutsu known to us in Naruto. Being a user of all five nature types, Tobirama was exceptionally strong. However, his true strength lied in the Jutsu that he created himself, such as the Shadow Clone Jutsu.
Like his Season Pass 1 counterpart, Minato Namikaze (Reanimation) is capable of using Flying Raijin Jutsu, which makes it possible to cover wide areas from long distances. He is adept at everything from high mobility, to taking on groups of enemies, to protecting his allies.
Being the reincarnation of Asura Otsutsuki, Naruto was offered half the power of the Sage, while Sasuke was given the other half. Upon returning to consciousness, Naruto had gained the incredible ability of the Six Paths Sage Mode.
While many associates the Flying Raijin Jutsu with Minato Namikaze, it was actually Tobirama Senju who created this unfettered ability.
Minato, rasengan creator, hailed as the fastest shinobi of his time and a better FTG user than Tobirama, its creator.
Minato's name was forever held in legendary light and he went on to further solidify himself as one of the strongest in the Fourth Great Ninja War after being brought back by Orochimaru using Edo Tensei.
The Flying Thunder God technique was arguably Minato's strongest jutsu. It is a useful space-time ninjutsu that grants the user the ability to instantaneously teleport.
6 BETTER: Flying Raijin Would Allow Kakashi To Teleport & Make Short Work Of His Foe. The Flying Raijin technique, also known as the Flying Thunder God, is a powerful space-time ninjutsu developed by none other than Tobirama Senju. It was later improved upon by Minato Namikaze, the Fourth Hokage.
Minato Namikaze
Also renowned as the Yellow Flash, Minato created the Rasengan at some point in his life.
I've seen others say its faster than light, then others say its only massively hypersonic+ then some even say its instantaneous. The most broken ability in the anime where you can spam a thousand seals or put it into kunai knives and throw it everywhere and teleport to where you want to be.
Ninjas who reached extraordinary heights were given the title of "God of Shinobi." Throughout the series, only two ninjas were given the title of God of Shinobi: Hashirama Senju and Hiruzen Sarutobi.
The Chidori was created by Kakashi Hatake after he failed to apply his lightning-nature to the Rasengan. He intended for the Chidori to be a weapon used only to protect friends and loved ones.
1 Kaguya Otsutsuki
Truly, Kaguya was a master of Taijutsu.
Sage of Six Paths' powers also granted him blinding speed, and he is also capable of using Space-Time Ninjutsu. Sasuke, with two of the most powerful Dojutsus at his disposal, can defeat Minato.
1 The Yellow Flash, Minato Namikaze
Minato even possesses greater control over the technique, using it to earn the nickname The Yellow Flash. He is considered the fastest shinobi of all time and gained worldwide recognition for his abilities.
Madara was one of the few shinobi with reflexes faster than Minato's.