Coined punningly after, respectively, teenage, teenager and teenaged, the adjective and noun jean-age, the noun jean-ager and the adjective jean-aged refer to the teenage years regarded as an age at which jeans are often worn.

The noun jean-ager occurs, for example, in Are you ever too old to wear jeans?, by Cate Dalton, published in the Sunday Mercury (Birmingham, West Midlands, England) of Sunday 27^th October 1996 [page 28, column 2]:

IT’s the copper riveted question we all have to face at some time. Am I too old to… wear jeans?
Like dancing to rock and roll in public or sending Valentine cards it’s one of those issues which not only defines the onset of a certain dignity but can lead us into treacherous self-delusion.
More and more people are refusing to part with their Levis just because they have run out of the Thirties.
In fact one in every four pairs of jeans are bought by somebody over the age of 40.
And jean manufacturers are cashing in on the older generation.
[…]
● What do you think? Are you still a jean-ager? Or did you fold away your denims and decide on something a little more gracious. Write to Jeans, Cate Dalton, Sunday Mercury, Birmingham, B4 6AZ.

—Cf. also the nouns adultescent and screenager.

The earliest occurrences that I have found of jean-age and of jean-ager are as follows, in chronological order:

1-: From one of the unconnected paragraphs making up the column This ’nd That, by Alta Heacock, published in the Waurika News-Democrat (Waurika, Oklahoma, USA) of Friday 4^th October 1946 [page 4, column 3]:

A very apt expression: “The jean-age girls.”

2-: From the Chicago Daily Tribune (Chicago, Illinois, USA) of Sunday 17^th November 1946 [Part 3 (Metropolitan Section): page 10, column 6]:

CROONER ‘SENDS’ JEAN-AGE SET TO ANOTHER WORLD
BY MARGUERITE RATTY

About a baker’s dozen of Mary Ann Owens’ feminine classmates ruled she couldn’t belong to their secret club unless she took them to an airing of ABC’s Breakfast club, which features her dad, Jack, as singer.
Mary Ann, 10 year old pupil at Winnetka’s Faith, Hope, and Charity school, turned an initiation dare into a delightful morning. After the broadcast Jack treated the girls to scrambled eggs, so we tagged along to learn, among other things, how you can make a love song hit the heart as early as 8 a.m.

3-: From the Chicago Daily Tribune (Chicago, Illinois, USA) of Sunday 1^st December 1946 [Part 3 (Metropolitan Section): page 22, column 4]:

TELE ARRIVES IN THAYER STREET; IT MAKES A HIT!
‘I’ll Always Remember This Day,’ Says Girl
By LARRY WOLTERS

This story, which might be titled “Television Turns the Corner . . . Into Thayer Street,” began with a telephone call from one of our jean-agers.
“That television stuff is here,” she began. “It’s in three big boxes. When can we open it?”

4-: From the following advertisement, published in the St. Petersburg Times (St. Petersburg, Florida, USA) of Friday 20^th December 1946 [page 2, column 7]:

“GOLD RIVET” JEANS
Deep tone denim dungarees to thrill the heart of any “Jean-ager.” Authentic Western style. Trim fitting!

5-: From the Chicago Daily Tribune (Chicago, Illinois, USA) of Thursday 16^th January 1947 [page 16, column 5]—Frederick Chase Taylor (1897-1950) was Colonel Lemuel Q. Stoopnagle in the radio comedy duo Stoopnagle and Budd:

COL. STOOPNAGLE IS SERIOUS, BUT NOT FOR LONG
Reverts to Character After W-G-N Show
BY LARRY WOLTERS

F. Chase Taylor was moderately serious last night refereeing that W-G-N-Mutual free for all for jean-agers, It’s Up to Youth. Mr. Taylor came out from South Norwalk, Conn., to take over the M. C. job on this show, broadcast from W-G-N studios. But as soon as he got away from the mike he swiftly reverted to Col. Stoopnagle, the character he’s been living on the air, in books and even in a good bit of private life for 20 years.
For two decades Col. Stoopnagle has specialized in nonsense, in fantasy and fiddle-faddle. His most delightful bits are contained in his Fictionary [unabashed]. Last night, inspired by his contact with Chicago high school youth, he entered a few more “daffynitions” in his book.

The earliest occurrences that I have found of jean-aged are as follows, in chronological order:

1-: From the column Deadline Chatter, by Lyle W. Nash, published in The Pasadena Independent (Pasadena, California, USA) of Friday 30^th March 1951 [page 3, column 1]:

THIS MORNING’S counsel is for you—the “jean-aged set.”
Any other free riders—like parents, for example—may get burned. You can skip right down to the bottom now and be sure of enjoying your coffee.
In our man-about-town tours we’ve found you teensters are facing some plenty rugged problems.

2-: From one of the unconnected paragraphs making up the column Flights of Fancy, published in the Catholic Digest (St. Paul (Minnesota): The Catholic Digest, Inc.) of September 1951 [Vol. 15, No. 11, page 20, column 2]:

Jean-aged girls.—Alvey E. Brown.

3-: From an advertisement for the department store Sears, Roebuck and Co., published in the Honolulu Star-Bulletin (Honolulu, Hawaii, USA) of Wednesday 19^th December 1951 [page 6, column 5]:

Pocket the 23c savings on these jean-aged favorites . . . better hurry and get them for Christmas! Husky 8-oz. navy blue denim with bright orange stitching, shiny copper rivets at strain points.