In Jackson's movie Aragorn tells Eowyn he is 87 years old, but in Tolkien's book, Aragorn's age is not mentioned.
Or is it? And if it is, how old is Aragorn when he fights at Helm's Deep, at the end of the Two Towers?
Aragorn was born on 1 March 2931 Third Age. The Battle of the Hornburg took place on 3 March 3019 Third Age, making Aragorn 88 years old (and 2 days) at the Battle of Helm's deep.
The Third Age turned into the fourth age in the year 3021, when Aragorn assumed the throne at the age of 90. He died, thus vacating the throne to his son Eldarion, in 120 Fourth Age at the age of 210. He, like all the descendants of Isildur, were long-lived for men.
I don't remember if it is actually mentioned in the texts, but his age is mentioned in the Appendices at least.