The common belief is that the harsh weather on Tatooine combined with the emotional turmoil he experienced during and after the Great Jedi Purge simply took a toll on his body. While that would be enough to age anyone, there may be a more viable answer.
Thanks to Obi-Wan's fight with Vader, it's possible that his old appearance in Star Wars: A New Hope is from the Dark Side of the Force, his burns causing his health to deteriorate, or some combination of the two.
Obi-Wan Kenobi is set 10 years after the events of Revenge of the Sith. Therefore, Obi-Wan is 48 years old in his solo show. As a comparison, Ewan McGregor was 50 years old when filming for the Obi-Wan Kenobi series began. A New Hope, the movie used as a reference for the Star Wars timeline, takes place in 0 BBY.
Many are often shocked to find that Obi-Wan was only 57 years old when he died fighting Darth Vader in A New Hope. For many, the issue of Obi-Wan's age is a plot hole, as he was initially seen as this old solitary man, but Star Wars made him younger than he looked in the original trilogy.
What is the age difference between Obi-Wan and Darth Vader?
It's a different story in A New Hope when Obi-Wan is older and slower - given this is ground zero for the Star Wars timeline, taking place in 0 BBY, it means Darth Vader is 41 years old at this point, while Obi-Wan is 57 when he dies.
Their last mission together to settle a trade dispute on Naboo set events into motion that would change the galaxy forever. Obi-Wan was 25 years old when he took the rank of Jedi Knight, and with it, Anakin Skywalker as his Padawan.
While it seemed like Anakin had lived a long life, his choices eventually led to his demise at 45. Though half of his life was filled with tragedy, he managed to make things right with the time he had and gained a Jedi burial that had him reunite with his old Master and Yoda.
A popular fan theory says that Obi Wan, who had feelings for Satine, fathered a child with her many years ago, though he doesn't know it. Corky Kris, who is introduced as satine's nephew, is actually that child, according to many. Though some say that Obi Wan would never.
Legend had it that Yoda—a Jedi who became Grand Master—was trained by N'Kata Del Gormo. A Force-sensitive Hysalrian, N'Kata Del Gormo was trained in the ways of the Force and achieved the rank of Master within the Jedi Order.
Kylo was born in 5 ABY (After the Battle of Yavin). The battle occurred at the end of Star Wars: A New Hope and marked the destruction of the first Death Star. The Force Awakens was said to have taken place in 34 ABY, making Kylo 29 years old when he made his onscreen Star Wars debut.
So Mace Windu was born around either 86 or 83 BBY making him 54 years old when we meet him in The Phantom Menace and 67 years old when he died in Revenge of the Sith.
Why does Obi-Wan Kenobi age like that? Because he's not real, and is a fictional character played by different people. You see, Obi-Wan Kenobi is played by Alec Guinness in the original trilogy. Guinness was 62 when he filmed A New Hope, and he died in 2000.
The Obi-Wan Kenobi is next in the Star Wars timeline, taking place nine years exactly ten years after Obi-Wan left his former apprentice for dead on the lava fields of Mustafar. This means Darth Vader is 32 in Obi-Wan Kenobi (his master, in contrast, is 48).
Grandmaster Yoda was so skilled with this power that he could heal himself or all in his immediate vicinity with just a wave of his hand. Near the end of his life, Obi-Wan Kenobi also developed this skill. Grogu using the Force to heal Greef Karga's arm wound.
Padmé was born in 46 BBY (Before the Battle of Yavin) and Anakin's miraculous Force-created birth came in 41 BBY, so Padmé is five years older than him. They do not meet until Star Wars: Episode I - The Phantom Menace, which takes place in 32 BBY.
Anakin was 23 yrs old when he turned to the dark side (19BBY) and his children were 23 when they defeated him. He was 46 at the time of his death (4ABY), meaning he also spent 23 yrs as Vader. At 23, he chose the dark, while at 23, his children did the opposite by embracing the light and turning him.
While he had suffered fatal injuries from the Emperor's lightning, Vader's redeeming final act restored his consciousness to Anakin Skywalker, and he had one last moment with Luke before peacefully accepting his death.
Luke Skywalker (portrayed by Mark Hamill) is the older twin brother of Leia Organa, the brother-in-law of Han Solo, the uncle of Ben Solo, the son of Anakin Skywalker and Padmé Amidala, and the grandson of Shmi Skywalker.
Yoda then peacefully dies at the age of 900, his body disappearing as he becomes "one with the Force". He leaves Luke with the knowledge that "there is another Skywalker." Moments later, Obi-Wan's spirit helps Luke come to the realization that the "other" of whom Yoda spoke is his twin sister, Princess Leia.
Obi-Wan Kenobi's third lightsaber was his new weapon that he created during the Clone Wars upon his promotion to Jedi Master and wielded until his death after his rematch with Darth Vader on the Death Star.
During Mandalore's Civil War and his apprenticeship under Qui-Gon Jinn, Obi-Wan fell in love with the young Duchess Satine Kryze. The two lived on the run for over a year. Satine reciprocated his feelings as he saved her life from insurgents and bounty hunters.
