In this species the sexes are described as "outwardly alike," meaning that the sexes have the same plumage and we can't tell the sex of the bird without some disassembly.*
But we can observe that the bird is in molt and thus tell a bit more about its age.
In the photo above, note some feather sheaths on the face (A). These are new growing feathers, not yet fully developed. Also notice that the new breast feathers are especially wispy with whiter tips (B) that contrast with the browner feathers of the shoulder. [You can click on the photo for a larger view.]
As these feathers wear and age, the plumage will become smoother and more evenly colored, as in a previous discussion of Cedar Waxwings.
Perhaps more obvious, we can see in the above photo that the outer tail feather on this bird (C) is short, and has not yet reached its full length. Wing and tail feathers in most birds are replaced sequentially so that the bird can continue to fly during the molt period.
Again, in the above photo, note that the 9th primary feather (D) is only half the length of the others and has more growing to do. [Waxwings have 10 primaries, but the outermost, the 10th, is always very tiny.] The new yellow belly and side feathers (E) are fresh and wispy. While the plumage of waxwings is naturally fluffy, these tips will wear down as the year goes on, creating a more smooth-looking plumage. The name-sake waxy tips of the secondaries (F) indicate this is an adult.
We thus can tell that this bird is an adult, 2 or more years old, undergoing a full pre-basic molt.
For more on molt and aging see this previous post.
* - Males have, on average, more extensively darker throats. Look at this web page that Mike Patterson alerted me to.
