It probably depends on when you believe a girl is considered an adult or a woman. If we take puberty as that point, since some describe it as the path to adulthood, then this Yahoo Answers Best Answer can give us the approximate age at which a girl has entered adulthood and become a woman.
Growth starts winding down at around age 17, but can continue until your early twenties.
Acne can occur at any age, but it usually starts getting less intense at around age 18.
Let us also consider that the Incubators choose girls because of their varying emotional states that are needed to produce the energy they need for the universe. (Kyubey explains to Madoka in Episode 9 that his kind can't make the energy themselves because they don't feel emotions.) The best time to do this is when a girl enters puberty, as this is when hormones are released into the body with one of the results being the heightened emotional state.
So with this in mind let's say the age range for a "girl" is between 8 and 20.
With this, the closest I can find is one person, Tsubaki Mikoto from Puella Magi Suzune Magica as the Wiki lists her age as "Around 20"
The Wiki also says under "Known Relatives"
Suzune Amano, her adopted daughter; she was also the surrogate mother for the Hinata twins.
If we take the adoption we know Tsubaki is at least 20 or 25 years old depending on the type of adoption Suzune falls under as explained here:
Age Requirements: In special adoptions, prospective adoptive parents must be over 25 years of age. In regular adoptions, prospective adoptive parents must be at least 20 years of age.
Also with the surrogacy we can probably assume that Tsubaki only raised the twins as Matsuri Hinata's page says
Tsubaki Mikoto, her deceased surrogate mother; her father and her deceased mother.
Not much is known of Matsuri's home-life. She had a sister, Kagari Hinata, an unnamed father, and a caretaker named Tsubaki Mikoto. Not much is given on how Matsuri relates to her family before the events of Suzune.
So depending on Tsubaki Mikoto's actual age and when our definition for when a girl reaches womanhood, Tsubaki is probably the only Magical Girl (from whom I can find an age) to become an adult.
NOTE: As pointed out on the wiki page:
(Note: As all chapters post-volume 1 are currently only available in Japanese, most of the post-volume 1 info below comes from rough translation and interpretation of events. Thus, some of the information may be incorrect, so please do not treat anything as 100% accurate.)