Juv chaffinches can be tricky (for a trainee) to age if they have replaced all of their greater coverts with adult feathers so it was really good practice for me ( and I got them all right !) - only three juv birds had at least one old greater covert. Key points to look for were 'dodgy' tertials, though some adult birds (Euring 6) had really 'punk' washed out tertials, and some of the 5's were replacing their tertials with adult feathers. Tails are also good for Chaffinches (as per Svensson) but funny things happen with tails also - a young bird may have replaced all, or some of its tail with adult feathers, or simply shat all over its tail in a bird bag making diagnosis (of shape) difficult ! A good tip for any trainees reading this is to look for any contrast between the greater coverts, tertials and primary coverts as your trainer has told you. A Euring 5 ( or bird hatched in 2007) will not normally replace its primary coverts until Summer 2008 so these feathers should be a different generation and will be worn on the tips and more pointy ( adult PCs are generally broad). The primaries themselves will look paler and have more wear. However, with Chaffinches watch out as adult male PC's naturally do not appear to be as 'glossy' black as the secondaries or primaries, and with females the difference can be subtle - I find with both it helps to 'close' the wing then look for any contrast, if there is none it is an adult.
I would encourage all serious birders to attend a ringing session. I have learned so much about plumage, moult, ageing ( and sexing). Can you sex a Goldfinch through your bins or scope ? What I have described about the wing feathers above is often possible to see from some photographs. For example, most of the recent photographs of the recent 'Dutch' Parrot Crossbills on the web are of 2cy birds Euring 5 eg. hatched 2007. Can you see why ? If not then consider doing some ringing or at least get a copy of Svennson - I am told all serious birders already have it ! ?
A ringer doing his thing and er.....looking at feathers on this Greenfinch.