In the manga and Sailor Moon Crystal, he is 3 years older than Usagi, meaning that at the start of the series (when she is 14), he is 17. He flashes her his high school student ID, which indicates he is a second-year student (out of the three years Japanese high schools last).
In the 90s anime, he's a college student, but his exact age is somewhat ambiguous. Since he drives both a car and a motorcycle, it's implied he's at least 18 years old (the legal driving age in Japan). In the final season, when Usagi says she is 16 years old and she's just entered high school, Mamoru is accepted to college in the United States--at Harvard. This means he's likely 18, if not older, perhaps 19 or 20, by series' end.
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Darien (Mamoru Chiba in the original Japanese) never cheats on Serena (Usagi Tsukino). He does break up with her toward the beginning of the second part of Season 2, in episode 61, "Huge Shock for Usagi! Mamoru Declares a Breakup!" (episode 55 in the dub, "The Cosmetic Caper"). During various points during their split, Mamoru deliberately led Usagi to believe he had moved on and gotten a new girlfriend (Unazuki/Lizzie, the younger sister of Mamoru's best friend, Motoki/Andrew), but she later found out that Mamoru had just done Unazuki a favor, and was not actually dating her.
Mamoru is in his early 20s in the anime.
Asana Mamoru is 171 cm.
Mamoru Uchida was born in 1900.
Mamoru Sato was born in 1937.