At the end/climax of Solo: A Star Wars Story, it is shown that Qi'ra is conversing with Darth Maul. If I recall properly, Darth Maul was cast in the Episode I: The Phantom Menace. It was the time when Anakin Skywalker was a youngling. His master, friend, and brother Obi Wan Kenobi killed Darth Maul in the climax of The Phantom Menace after the fall of the latter's master Qui-Gon. Then how come is he present in the times of Solo: A Star Wars Story? Is Han Solo quite older compared to Luke And Leia? I am having serious confusion thinking of the timeline.
Han Solo starts 'Solo' at age 19, and then the story jumps ahead three years when Han is 22 years of age.
Solo: A Star Wars Story
Alden Ehrenreich portrayed a younger Han Solo in Solo: A Star Wars Story. Main article: Solo: A Star Wars Story A film featuring Han Solo before the events of the 1977 film was released on May 25, 2018, starring Alden Ehrenreich. In the film, 19-year old Han is characterized to be an orphan on the planet Corellia. He and his lover, Qi'ra (Emilia Clarke), attempt to escape from a criminal gang and bribe an Imperial officer with a stolen sample of coaxium, a powerful hyperspace fuel, in exchange for passage on an outgoing transport, but Qi'ra is apprehended before she can board. Han vows to return for her and joins the Imperial Navy as a flight cadet. He is given the surname "Solo" by the recruiting officer, apparently referencing Han's statement that he "has no people".
Three years later, Han has been expelled from the Imperial Flight Academy for insubordination.
In addition Darth Maul survives being cut in half by Obi-Wan in The Phantom Menace, becoming just "Maul" continuing his story on in Star Wars: The Clone Wars, Solo: A Star Wars Story, and eventually dying in Star Wars: Rebels.
By the time of 'Solo', Maul has become the leader of Crimson Dawn, which is solidified at the end of the film.
Yes, Han Solo is about a decade older than Luke & Leia Skywalker.
For the full time line between the Prequels and OT, please see my answer with source on this question.
According to Solo: The Official Guide, Han Solo is stated to have been born in 32 BBY. He is somewhere between 19 and 22 in the movie.
As for Darth Maul, if you watch the credits, he is credited only as Maul, no Darth anymore, he is not a Sith Lord anymore, he had been replaced by Darth Vader. And it seems that you don't follow spin-off materials, Maul has a long run in these, he rebranded himself as a crime lord, and if you look carefully he has robotic spider legs.