Mari's age is a complicated one...but since she has the curse of Eva, the same rules apply to her as it does with the Spirits in Date A Live, she appears 18, but her chronological and biological age is heavily implied to be much older—likely in her late 40s or 50s due to the Curse of Eva which stops physical aging.

Also, for those who are wondering where I got this idea from? Check out Sachik0Sama image they made of Mari, you'll understand why.