--Myrtle Wilson is described as being "in her middle-thirties and faintly stout" (Chapter Two, page 35, Scribner Classics edition).
--George Wilson's age is not given, but Nick narrates that he and Myrtle have been married "twelve years" (Chapter Eight, page 134), which would put Myrtle in her early to mid-twenties at the time of their wedding. If her husband were at least Myrtle's age or a little older than she at the time of their marriage, which was sometimes the case in the era of the novel, George would be somewhere between his middle and late thirties.
--Jay Gatsby is described by the narrator, Nick Carraway, when he meets Gatsby at the party to which he has been invited. Nick mentions that, as he talks with Gatsby, Gatsby's impressive, reassuring veneer "vanished." Instead, Nick finds himself looking at "an elegant young rough-neck a year or two over thirty" (Chapter Three, page 52).
Gatsby is really the only character in the novel whose age is actually hinted to by Nick. Nick goes through a detailed description of Gatsby and his "elaborate" way of speaking around chapter four. In this chapter, Nick specifically states that Gatsby is "a year or two over thirty." The reader can assume from this that most of the other characters are of the same age because they mention that they all went to college at around the same time (Nick, Tom). However, there is no evidence in the novel to tell the actual age of Myrtle or George Wilson. The reader can assume that they too would be around the same age as Gatsby -- thirtyish -- but this information is never provided.
When Nick first sees Myrtle when Tom took him to Wilson's garage, Nick says she is a woman in her mid-thirties.
"His voice faded off and Tom glanced impatiently arounf the garage. Then I heard footsteps on a stairs, and in a moment the thickish figure of a woman blocked out the light from the office door. She was in the middle thirties......"
This quote was on the lower half of page 25 of my book, not sure if you have the same copy.