23? 

Harry finds a bunch of useful information in Sirius' and Regulus' old bedrooms at 12 Grimmauld Place (the place that is too dangerous to visit, because everyone knows the secret, remember?)

By an amazing miracle, both bedrooms are completely intact! Yep - Regulus' parents didn't get rid of any of his stuff, even though he's been dead for 18 years. Nor Sirius', even though they burned him off the tapestry and disowned him. And of course nothing was thrown out when Molly and the rest of the Order cleaned the entire house.

Harry finds a baby picture of himself that Lily sent to Sirius - funny how Sirius never thought to dig that out and show it to Harry during any of the months Harry lived at 12GP.