No specific mention is made of how old Juliet's parents are in Romeo and Juliet. However, when Juliet and Lady Capulet are talking in Act I, Scene III, Lady Capulet does say "By my count / I was your mother much upon these years / That you are now a maid," meaning that Lady Capulet had already given birth to Juliet at age thirteen, the age that Juliet currently is (77-79). This would make Lady Capulet roughly twenty-six years old.
Figuring out Capulet's age is a little more difficult. It is clear that he is considerably older than his wife, but as with Lady Capulet, there is no specific mention of his exact age. The best scene to use in trying to figure out his age is Act I, Scene V. As the family prepares for the masquerade party, Capulet says "I have seen the day / That I have worn a visor and could tell / A whispering tale in a fair lady's ear, / Such as would please. 'Tis gone, 'tis gone, 'tis gone," meaning that he is no longer of the age to flirt with or court the young ladies at the party (25-28). Shortly afterward, he tells his cousin that they should sit, stating "[f]or you and I are past our dancing days" (36). Immediately following, he asks his cousin how long it has been since they wore masks at a party, and his cousin replies that it has been "thirty years" (39). This shocks Capulet, and they argue about it. Capulet claims they both wore masks at Lucentio's wedding, which was twenty-five years before, but his cousin replies that Lucentio's son is already thirty years old (41, 45). In other words, it has been at least thirty years. If the normal age for wearing masks/courting is late teens through late twenties, as is the case with Romeo and Paris, then this would make Capulet between about forty-eight and fifty-eight.