In the original West End and Broadway musical productions of "Mamma Mia!", which opened in 1999 and 2001 respectively, Donna is generally portrayed as being in her late thirties. This is evident through various cues in the script, such as the song "Does Your Mother Know" in which Donna sings about dating a younger man, Sam, and the fact that her daughter, Sophie, is about to turn 20 years old.
However, in the film adaptation of "Mamma Mia!", released in 2008, Donna's age is adjusted to be in her early forties. This change was likely made to accommodate the casting of Meryl Streep, who was 59 years old at the time of filming, in the role.
Therefore, depending on the version of "Mamma Mia!" you are referring to, Donna's age can range from late thirties to early forties.