By the time of Star Wars Rebels, Ahsoka was an adult and someone well-trained in the Force. She began her journey in the Star Wars saga as Anakin Skywalker's Padawan in Star Wars: The Clone Wars - first in the animated movie and then later in the animated series, which only recently completed its final season - and continued fighting tyranny in the galaxy well through the Galactic Civil War.
Unlike most of the people she fought alongside, Ahsoka was just a teenager throughout the Clone Wars - she was, after all, a Padawan, just like Anakin was in Star Wars: Episode II - Attack of the Clones - yet she carried herself as a Commander in battle, time and time again. Her age had nothing to do with her skills in combat, evidenced by the fact that she beat Darth Maul on Mandalore, but it was something that was contested by those who looked down on Jedi leading the Clone Wars.
Ahsoka was 17 years old when the Clone Wars ended and she disappeared shortly after Palpatine enacted Order 66. While she roamed the galaxy for the next few decades, operating as Fulcrum for the Rebel Alliance, Ahsoka didn't reappear on-screen until the events of Star Wars Rebels, which took place only a few years before A New Hope. Since Star Wars Rebels begins five years before the Battle of Yavin, which itself is 19 years after Star Wars: Episode III - Revenge of the Sith, that makes Ahsoka 31 years old in season 1.
Since Ahsoka is 31 years old when Star Wars Rebels starts, that means she's 36 years old when it ends - at least when the core story ends in the lead-up to both the Battle of Scarif in Rogue One: A Star Wars Story and the Battle of Yavin in A New Hope. However, Star Wars Rebels' actual ending features a flash-forward sequence showing Ahoska and Sabine Wren after the Battle of Endor, which was four years after the first Death Star was destroyed. That means, all in all, Ahsoka was 40 years old the last time Star Wars fans saw her on-screen.
Of course, Ahsoka's voice was heard in Star Wars: The Rise of Skywalker, which seemed to indicate she was dead. While that notion has been disputed, it stands to reason that she still did die sometime before the Star Wars sequel trilogy began. But soon fans will get to see Ahsoka in live-action in The Mandalorian season 2. Since that show takes place five years after the events of Return of the Jedi, Ahsoka will be 45 years old, unless the new season has a time jump.