WARNING: Spoilers for Action Comics #1036 ahead!
There have been many changes and updates to Superman's looks over the decades, but in the recent Action Comics #1036, it is confirmed that Kal-El's newest change is here to stay. Moving forward, Clark Kent is going to have grey hair, acknowledging the aging Man of Steel. With Jon Kent taking over as Metropolis' new Superman, and Kal-El off-world fighting against Mongul, this new look proves that even Superman isn't immune to the perils of old age.
There have been many instances throughout DC continuity where Superman is seen in the far future - whether in The House of El or various other Elseworlds - and Kal-El is usually depicted with gray hair in those glimpses. However, the current redesign of Superman has been confirmed to not be in an alternate timeline or Omniversal Earth. From now on, Clark will be depicted with graying hair, acknowledging his role as an elder statesman of the Earth's superheroes. It has been made clear that this is not the end of Superman, but, in fact, the start of a new era, one where Superman is able to recognize his past flaws and weaknesses and grow from them.
In the recent miniseries Superman and the Authority, it appears that in an alternate universe Kal-El is losing his powers, growing old and grey, and is desperate to save the world by reforming the antiheroic group The Authority. He gathers a team of metahumans including Manchester Black, Enchantress, Apollo, and Midnighter, eventually revealing that this team will be traveling to Warworld to defeat Mongul. However, in Action Comics #1036 - illustrated by Sami Basri and Daniel Sampere with writing by Sean Lewis and Phillip Kennedy Johnson - it has since come to light that this was actually not an Elseworlds story and that it is very much happening in the current continuity of Superman in Infinite Frontier.
At the beginning of Kal-El and the Authority's assault on Mongul and Warworld it appears that Superman is in the prime of his health and power. This vision of Superman is nothing more than a powerful illusion, propped up by his new teammates Manchester Black and Enchantress. Black lets the rest of the team in on Superman's secret: his power has been waning for months after a brutal exposure to radiation poisoning, so he enlisted Black's telepathy and Enchantress' magic to boost his power levels and give the appearance that he is at full strength. This illusion is very quickly broken by Mongul, who exposes Superman as having the greying hair and weakened power that was seen in Superman and the Authority.
While a few grey hairs might not seem like a big deal, showing that Superman is now aged goes a long way to illustrate that the world is entering into a new era of The Last Son of Krypton. The once invulnerable Kal-El has been shown to be weaker than once thought, and his son Jon is now the idealized version of Superman that Clark once was. Superman rarely changes as a character, so seeing him with a new design and weakened powers in Action Comics #1036 opens him up to new and complex storylines that can put a fresh twist on the classic hero.