User blog comment:B214/Question and Answers/@comment-5882552-20130129053041/@comment-211.28.133.153-20130129121653

Rias's age isnt stated in the LN volumes, but in Dragon Magazine it says that she is 18 years old. Though Im not quite sure on the actual age of the other girls in Rias's peerage. In volume 8 SS its stated that Ise is 17. In volume 7 afterword its said that the setting of Rossweisse's age is 17 to 18.

Also Sirzechs is alteast a hundred years old since he made Okita Souji his servant.