In act 1, scene 2 of William Shakespeare's Romeo and Juliet, Lord Capulet, Juliet's father, tells Paris, one of Juliet's suitors, that Juliet "hath not seen the change of fourteen years" (1.2.9). In fact, according to Juliet's mother and Nurse in act 1, scene 3, Juliet is a little more than two weeks shy of her fourteenth birthday. Lady Capulet states, "She's not yet fourteen" (1.3.14), and Juliet's nurse, who's been intimately involved with Juliet since the day she was born, states definitively that in a little more than two weeks, "Come Lammas Eve at night shall she be fourteen" (1.3.21).
There is no such direct evidence of Romeo's age anywhere in the play. It seems odd that Shakespeare makes an effort to clearly establish Juliet's age—an age which is notably younger than the average age for a woman to get married in Shakespeare's time—but Shakespeare never mentions Romeo's age.
In one of two major sources for Shakespeare's Romeo and Juliet, Arthur Brooke's narrative poem "The Tragicall Historye of Romeus and Juliet," published in 1562, Juliet "Scarce saw she yet full sixteen years: too young to be a bride!" (line 1860), but nothing is said about Romeo's age except "Upon whose tender chin, as yet, no manlike beard there grew" (line 54).
In the other major source for Shakespeare's play, the story of "Rhomeo and Julietta," novel 25, in William Painter's The Palace of Pleasure, published in 1567, Julietta is "a yong Gentlewoman of Verona," no age given, and Rhomeo, "of the family of Montesches," is "of the age of 20 or 21 Yeares."
Shakespeare's Romeo seems younger and more immature than Painter's Rhomeus of "20 or 21 Yeares." Nevertheless, Painter's Rhomeus, like Shakespeare's Romeo, falls in love with Julietta at first sight, impulsively marries her, kills Tybalt, is banished from Verona, goes to live in Mantua where he he's told, mistakenly, that Julietta has died, buys poison, and returns to Verona determined to kill himself at Julietta's tomb.
Back in Verona, Rhomeo finds Julietta seemingly dead in her tomb, whereupon he
drew the Poison out of his box, and swallowing down a great quantity of the same, cried out: "O Iulietta, of whole the World was unworthy, what Death is it possible my Heart could choose out more agreeable than that which yet suffereth hard by thee? what Grave more Glorious, than to be buried in thy Tombe? what more worthy or excellent Epitaph can be vowed for Memory, than the mutual and pitiful Sacrifice of our lives?"
Whether he's twenty-one years old, as in Painter's "Rhomeo and Julietta," or in his mid-teens, as he seems to be in Shakespeare's Romeo and Juliet, Romeo is still the same romantic, love-struck, immature, impetuous, passionate, and reckless "star-crossed lover" who, with Juliet, through their "misadventur'd piteous overthrows" and the "fearful passage of their death-mark'd love," by their deaths "bury their parents' strife" (Pro. 6–9).