When Beth is adopted in The Queen's Gambit, the orphanage lies about Beth's age — so how old is she, and why lie in the first place? The Queen's Gambit is Netflix's hit miniseries based on the Walter Tevis novel by the same name; it's a coming of age story about a young American chess prodigy who learns to overcome her personal demons. The Queen's Gambit follows Beth from her childhood until adulthood — but is unclear about her exact age throughout.
Beth is orphaned in The Queen's Gambit when her mother dies in a car crash, which is implied to have been an intentional murder-suicide attempt. Beth's father, whose identity is never confirmed, is not in the picture, which leaves the young woman an orphan. She is sent to the Methuen Home for Girls, where she remains until she is adopted, as a teenager, by Alma and Allston Wheatley.
When Beth first meets the Wheatleys, the director of the orphanage, Helen Deardorff, tells them that Beth is 13. Beth starts to correct her, beginning to say she's fifteen, but Deardorff stops her. Likely, the director was operating under the assumption that the Wheatleys wouldn't want an older child, so she lied about Beth's age; however, as Alma reveals later, she intentionally chose to adopt a teenager, having previously lost a child and (presumably) not wanting the additional responsibility required for raising an adolescent. Deardorff assumed that the older a young woman was, the less viable she was as a candidate for adoption — but Beth's case proves she was wrong, and for some prospective parents, a teenager is an ideal option.
The Queen's Gambit shows that Elizabeth was 9 years old when she entered the orphanage. Her birth certificate, seen in episode 1, shows her birthdate as "November 2, 1948," and was thus approximately 15 years old when she won her first tournament in 1963 — but not when she was adopted, since it appeared to happen in the summer. Chances are, Beth was about the say she was "fifteen in November" when Deardorff interrupted her, since the timeline doesn't make sense otherwise. Presumably, she told Alma her true age, which explains why she was able to graduate just three years later in 1966. Since Alma already admitted she didn't want a younger child, it's likely she would have been even happier to learn that Beth was already nearing adulthood (which further explains Alma's hands-off parenting style with Beth).
Beth is either 19 or 20 at the end of The Queen's Gambit, as the tournament in Russia takes place in 1968, but the time of year is not clear. Based on the fact that she's described as being 20, the event likely takes place late in 1968, after her birthday in November. There are some issues with Beth's age — namely, it makes Beth's relationship with Townes a little problematic given how young she was when he had her in his hotel room— but the fact that her age is kept vague helps focus the story on her personal development, and the various emotional milestones she reaches on her journey in The Queen's Gambit.