Shelly Johnson's Age

All evidence suggests that Shelly Johnson isn't much older than Audrey, Laura, or Bobby during the original series.

In season one, episode five (#1.005, Cooper's Dreams), Bobby tells Shelly the following:

According to Wikipedia, the average American is 16 or 17 in the 11th grade.

In the script for season one, episode four (The One-Armed Man), Shelly reveals the following while discussing Leo with Norma Jennings:

                               "He was so great at first, you know? This flashy guy in
                               his hot car; sure beat sitting in some stupid classroom ...
                               we get married and I find out what he's lookin' for is a
                               maid he doesn't have to pay ... I feel so stupid."

In the series, there is a pause in her speech after "This flashy guy in his hot car." Shelly's dialogue plays over Norma's reaction, indicating the section above could have been recorded. The dialogue further confirms that Shelly was in school when she married Leo.

When Hank Jennings first arrives at the Double R, he is unaware that Shelly is Leo's wife, meaning that Leo was unwed before Hank was sent to jail eighteen months prior.

In season one, episode five (Cooper's Dreams):

In season one, episode seven (The Last Evening):

The script for season one, episode four (#1.004, The One-Armed Man) supports the contention that Hank was sent to jail eighteen months prior.

                                       HANK
                               -That my time here has been good for me. You don't
                                spend eighteen months in a nine-by-five box without
                                facing your life. Without figuring what went wrong with
                                Your life before. Without planning to make it better.
                                Without learning to get down on your hands and knees
                                and pray for a second chance.

Mark Frost's recent novels also place Hank's incarceration as having taken place in 1987. The Secret Diary of Laura Palmer states the same.

The Star Pics trading card for Shelly Johnson says she was born 5/30/72. The card for Mark Frost in the same set states the following: Bats right - throws right. Coached the Twin Peaks Softball Team to a near perfect 24 - and 14 - record in the 1989 - 90 season which seems to indicate the Star Pics cards set the series in the year the cards were released, 1991 or at least '90. If the cards were set in 1989, Shelly would have only been 16, 17 in '90, or 18 in 1991. Shelly's card lists her weakness: "I should have stayed in school and gotten a bit more mature before marrying. I feel I have missed something."  The card also states she completed only two years of high school. According to Wikipedia the average age of high school students in their second year is 15 to 16.

In The Secret Diary of Laura Palmer, Shelly is first mentioned as Leo's wife in an entry dated December 23, 1987. However, in the diary, Laura lives through 1989, so it's almost impossible to use it as a means of dating matters.

The Mauve Zone website hosts many rare and interesting videos. One video contains deleted scenes from season two, including multiple takes of a scene in which Mark Frost portrays news anchor Cyril Pons. Pons makes a report concerning two victims suffering from smoke inhalation found at the Packard Sawmill Fire. He lists Pete Martell and Shelly Johnson, the latter he claims is 19. 

Mark Frost's 2017 Twin Peaks tie-in novel, The Final Dossier, states, 

"Corroborated testimony also suggests that this relationship with Bobby during high school predated Leo: Bobby and Shelly were, in fact, contemporaries who had known each other going all the way back to elementary school." Shelly is said to have been seventeen when she met Leo Johnson. "She dropped out at the end of her junior year when Leo Johnson - six years her senior - careened into her life." The Final Dossier details how Shelly and Bobby created a scene at the prom when she found out he was cheating on her with Laura. She left the prom for the Roadhouse, where she met Leo. Their marriage lasted for "nearly two years". One year to the day of Leo's death, Shelly married Bobby Briggs. Shelly was seven months pregnant at the time. Further on, Frost writes, "Within a year of Leo's death, when she turned twenty-one, Shelly now had a home, a husband she loved, a beautiful child, and a supportive circle of friends and customers at a job she adored." *

Using the information from the original series, Shelly was 16 or 17 when she dropped out of school to marry Leo. Eighteen months passed since Hank Jennings knew Leo was single, making Shelly 18 going on 19 - if she was 17 when she and Leo married.

According to this interview, Mädchen Amick was only 17 when she was accepted for the role of Shelly Johnson. 

Conclusion; going by what is said in the original series, Shelly was more than likely 18 or 19 during the original series.

 

*How can she have had a child within a year of Leo's death if it was one year to the anniversary of his death that she married Bobby Briggs, still carrying that same unborn child?

I admit to having difficulty accepting the information in The Final Dossier and The Secret History of Twin Peaks. I don't want such a statement to offend others. I can't comprehend some 'facts' such as Bobby dating Shelly before Bobby's relationship with Laura and her relationship with Leo. The story told in The Secret Diary of Laura Palmer is what is ingrained in my mind as truth. Laura was Bobby's first love. It corresponds with what is said in the television series.