Like practically all eroge, School Days does not list the ages of its characters anywhere. However, it is trivial to show that the characters are 18+.
School Days (and School Days HQ) is by 0verflow.
0verflow is part of Sofurin/the EOCS url=http://ja.wikipedia.org/wiki/%E3%82%AA%E3%83%BC%E3%83%90%E3%83%BC%E3%83%95%E3%83%AD%E3%83%BC_%28%E3%83%96%E3%83%A9%E3%83%B3%E3%83%89%29[/url]
Games released by EOCS members do not feature sexual activity involving characters aged less than 18 years of age url=http://ja.wikipedia.org/wiki/%E3%82%B3%E3%83%B3%E3%83%94%E3%83%A5%E3%83%BC%E3%82%BF%E3%82%BD%E3%83%95%E3%83%88%E3%82%A6%E3%82%A7%E3%82%A2%E5%80%AB%E7%90%86%E6%A9%9F%E6%A7%8B#.E3.83.AD.E3.83.AA.E3.83.BC.E3.82.BF.E7.B3.BB.EF.BC.8818.E6.AD.B3.E6.9C.AA.E6.BA.80.EF.BC.89.E4.BD.9C.E5.93.81.E3.81.AB.E5.AF.BE.E3.81.99.E3.82.8B.E8.A1.A8.E7.8F.BE.E8.A6.8F.E5.88.B6[/url]
THEREFORE all characters in School Days HQ save for characters like Itaru who are not featured in H scenes are 18 years of age or older.
EDIT: I can’t provide English language versions of the aforementioned sources.