We are not told Ellen's exact age in The Age of Innocence, but it is relatively easy to extrapolate an approximate age for her.  Since women at this time and in this level of society usually married at the age of twenty-one (and perhaps a year or two before or after, less commonly), and Ellen has been away for less than ten years, she is probably in her late twenties or early thirties.  She is about the same age as Newland Archer, who guesses in Chapter Eight that she is "nearly thirty" (59).  She had been in New York for a time in her childhood (of which Ellen and Archer both have memories), but she returns again to New York when she has left her husband Count Olenski after a period of a few years of marriage.

Ellen has an ageless quality about her, as noted by Archer:

In the middle of the room she paused, looking about her with a grave mouth and smiling eyes; and in that instant Newland Archer rejected the general verdict on her looks. It was true that her early radiance was gone. The red cheeks had paled; she was thin, worn, a little older-looking than her age, which must have been nearly thirty. But there was about her the mysterious authority of beauty, a sureness in the carriage of the head, the movement of the eyes, which, without being in the least theatrical, struck his as highly trained and full of a conscious power. (59-60)

Source: Univeristy of Virginia etext, The Age of Innocence http://etext.lib.virginia.edu