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The story doesn't give Harry's age directly. However, there's a sentence that states, "Not for the first time [Snape] wished he could get his hands on a timeturner, go back 15 years to Hogwarts, and just poison [Fred and George]." Fred and George were two years ahead of Harry, so that would make Harry somewhere between twenty-four (if Snape was thinking of going back to the twins' first year) and thirty (if he was thinking of going back to their last year).