Darth Maul was in his 50s when he died in Star Wars Rebels, but he was in his prime during the Clone Wars and throughout most of the Empire's reign across the galaxy. First appearing in Star Wars: Episode I - The Phantom Menace, Darth Maul unveiled the Sith to the Jedi after a thousand years in hiding. He fought and killed Qui-Gon Jinn on Naboo, which was quite the feat for the Sith Lord, but he failed to kill Obi-Wan Kenobi as well; instead, he was apparently killed by being cut in half.
Of course, that didn't last, as Darth Maul returned to the fold in Star Wars: The Clone Wars and became yet another foe for the Jedi and the Republic. His storyline not only delved deeper into his backstory but illuminated aspects of the Sith - and Palpatine, for that matter - that were left ambiguous by the prequel trilogy, including how much of his plan was known ahead of time. Darth Maul ultimately escaped the clutches of the Republic during Order 66 and lived to lead Crimson Dawn in the time period between the prequel and original trilogies.
Since much of Maul's backstory has been told outside of canon material, his exact age is questionable. According to the now-Legends book Jedi vs. Sith: The Essential Guide to the Force, Maul was born in 54 BBY. Going based on that date, Maul would've been 22 years old when he fought Obi-Wan on Naboo and 34 years old when he first appeared in Clone Wars. When Order 66 happens during events of Star Wars: Episode III - Revenge of the Sith and Maul disappears, he's 35 years old.
Teased in Clone Wars season 7 as well as Solo: A Star Wars Story, Maul ultimately became the leader of the syndicate Crimson Dawn during the Age of the Empire, and his journey ultimately brought him to the planet Malachor, where he met Ezra Bridger for the first time. That was in 3 BBY, thus meaning Maul was 51 years old when he first appeared in Star Wars Rebels season 2. His time on the show didn't last long, seeing as he was killed off - for the final time - in a duel with Obi-Wan on Tatooine in Star Wars Rebels season 3. He was 52 years old when he died in 2 BBY.
Maul was three years younger than his rival Obi-Wan, who was 55 years old when he killed the former Sith Lord at the tail-end of the Empire's rule. Comparatively, Ray Park, who's played Maul in live-action and performing motion capture for animation over the years, is 45 years old. So although he's still seven years younger than Maul was at his oldest, he was actually three years older than the Sith Lord in The Phantom Menace.