This article is from the rec.music.reviews FAQ, by Al Crawford (email@example.com) with numerous contributions by others.
Sure. There's no restriction that reviews have to be of new releases.
Feel free to write a review of a release that's 6 months, 6 years or
even 60 years old, there might well be someone out there who has been
contemplating buying it and would appreciate your review. It's advisable
though not essential that reviews of older material give some indication
as to year of release and availability though.