Today I retrapped a Treecreeper which was originally ringed in January 2013 and aged as a first winter bird (age code 5). According to the Identification Guide to European Passerines by Lars Svensson there is a “difference in the size and shape of the pale tip on the outermost long (3rd) primary covert, adults having smaller and more rounded tips, young birds larger and on average more drop-shaped or wedge-shaped pale tips”. However he does recommend more testing of this method.
Today's bird supports this aging method. Below is a juvenile bird for comparison.
juvenile Treecreeper wing (M. Pearson)
adult Treecreeper wing, age confirmed through ringing (M. Pearson)