Maddy Ferguson's Age

Madeleine Ferguson's age is never mentioned in the televised series, leading many to assume she is a teenager. This is more than likely based on the company she keeps and her closeness to her cousin, Laura Palmer. Maddy, however, is an adult according to all but one citing of her age.

From the script to episode three (mistakes included):

Leland looks up, sees the Nurse and MADELEINE FERGUSON standing before him.
Madeleine is twenty, quite beautiful, She wear glassses, has jet black hair worn long. She sets down a suitcase. A closer angle REVEALS that she looks very much like Laura Palmer.

Jennifer Lynch's The Secret Diary of Laura Palmer has Maddy four years older than Laura, who was twelve when she wrote the following: Maddy is so pretty now. She's sixteen years old and I'm so jealous of her life! I wish I were sixteen! She has a boyfriend at home that she already misses, and he called her at the house earlier just to make sure she got here all right. 

Maddy's Star Pics Trading Card states that her date of birth is June 22, 1968. Laura's birthday, as you'll remember, is July 22, 1972. Making Maddy four years older. 

From the card,

Accomplishments:
I have my a job and my own apartment in Missoula. I value my independence.

Strengths:
Laura and I were so close, and because I was the "older" cousin, I would often give Laura advice and moral support.

As stated in this post, I believe the Star Pics cards were set at the time of their creation, 1990 or '91. This is similar to how The Secret Diary of Laura Palmer continues through October 1989, though Laura died on February 24th, 1989 in the television series. 

Another age for Maddy is given in the script to episode four:

 MADELEINE
        Laura was a month older than me. Hadn't seen each
        other much recently. I live in Missoula. I'm a
        receptionist at a insurance company?
     
She states she is a receptionist, corresponding with the information on her Star Pics trading card, leading one to think she must have been older than Laura, despite what is said. 

In the script for episode fourteen (2.007, Lonely Souls), Maddy says the following to Sarah and Leland;

MADDY
(CONTINUED)
I've been thinking. I've really enjoyed my stay and everything, but I really feel like it's time to go home.

Sarah and Leland lower their papers.

MADDY
(CONTINUED)
To Missoula. I've got my job and my apartment and I just ... miss having my own life, so I think tomorrow I'll probably be driving back home.

Aside from a few moderate changes, such as Maddy saying, "A life of my own" instead of "My own life," the dialogue remains the same in the televised episode.

In summation, I believe Maddy was either 20 or 21 years of age during the airing of the original series.