Mr. Bingley had not been of age two years, when he was tempted by an accidental recommendation to look at Netherfield House. He did look at it, and into it, for half an hour... (Chapter 4).

If we know what was generally considered "Of age" for a Regency male, we will have our answer. [As Cheryl notes, Darcy's age is given near the end of the novel.]