Although Superman & Lois' Clark Kent (Tyler Hoechlin) has only been part of the Arrowverse since Supergirl season 2, the Man of Steel is one of the oldest and most experienced superheroes in the world. Superman is integral to his cousin, Kara Zor-El (Melissa Benoist), coming to Earth and becoming the Girl of Steel but he was only teased in Supergirl season 1. The Arrowverse has since retconned Superman's heroics into its history and the Man of Steel's life with his wife, Lois Lane (Elizabeth Tulloch), was retconned again by Crisis On Infinite Earths.
When Kara decided to follow in Clark's footsteps and become Supergirl, her cousin Superman was already considered the greatest superhero of Earth-38 for many years. Supergirl decided to base herself out of her hometown of National City but Superman was long-established in Metropolis by then. Meanwhile, Earth-1, where Green Arrow (Stephen Amell), The Flash (Grant Gustin), and the Legends of Tomorrow lived, had never heard of Superman or Supergirl until the Kryptonian cousins joined the annual Arrowverse crossovers. Crisis On Infinite Earths then destroyed the Multiverse and rebirthed it anew, folding Earth-38's history into Earth-1 so that Superman and Supergirl always lived on the same Earth as the other superheroes. But this massive change for Clark and Kara ended up altering their histories; Lois and Clark not only became the parents of twin boys but their sons, Jonathan (Jordan Elsass) and Jordan (Alex Garfin), aged up to 14 years old.
Superman & Lois episode 11, "A Brief Reminiscence In-Between Cataclysmic Events", filled in the revised origin of Clark Kent in the Arrowverse and clarified his personal timeline as Superman. Kal-El was born on Krypton (which soon exploded) in 1979, which makes the Arrowverse's Superman 42 years old in 2021. Of course, Clark's Kryptonian genes let him age slowly in Earth's yellow sun so he still looks a decade younger (Tyler Hoechlin is 33). Superman often refers to his "20 years of being a superhero", which makes him the most experienced of Earth's defenders (although it's an open question how long the Arrowverse's Batman, who Clark is friends with, operated in Gotham City prior to his disappearance before the events of Elseworlds and Batwoman season 1). Nevertheless, Superman has been saving the world, which is now the amalgamated Earth-Prime, for many years before Oliver Queen became the Green Arrow and kicked off his generation of superheroes, which includes Supergirl.
According to Superman & Lois episode 11, Clark left Smallville in 1995 and traveled north to create the Fortress of Solitude and train with his Kryptonian father, Jor-El (Angus Macfadyen). Kent then returned to Smallville in 2004 to reunite with Lana Lang (Emmanuelle Chriqui) but after he saw Lana had moved on romantically, Clark decided to move to Metropolis. Superman began making heroic saves in Metropolis soon after, around 2005 when Clark was about 26, which is also when Kent began working with Lois Lane at the Daily Planet. Clark was Superman for a year before he saved Lois as the Man of Steel for the first time, although she had no idea at that point that Superman was really Clark Kent. Meanwhile, Lois and Clark fell in love and he told her that he was Superman. They were married in 2006 and their twin boys, Jonathan and Jordan, were born 9 months later, likely in 2007.
If Superman debuted in 2005, there's an approximate 5-year discrepancy with Clark's count that he's been a hero for 20 years. The most likely solution to this issue is that Clark began performing heroic acts during his training with Jor-El. He wasn't officially Superman at that point, and he wouldn't receive his first costume, which was sewn by his adoptive mother Martha Kent (Michele Scarabelli), until his return to Smallville in 2004. Regardless, by 2021 Superman & Lois' Man of Steel is renowned and beloved as a veteran superhero who has saved the world countless times, and Kara Zor-El was inspired to model herself after her cousin Clark's example when she decided to become Supergirl.