Dickon is likely too high for you as he's a tenor, and um...Colin is more of a soprano. It's tricky as the best baritone part is probably Dr. Neville, but he's older, and Archibald could also be played by a baritone (with a strong/well mixed falsetto) but is also old.
Sorry I couldn't bring you better news. However, if you have a strong upper range I would shoot for Dickon. I can sing Dickons main two songs (Winter's on the Wing and Wick) well (or so I'm told), and I concider myself a high baritone. If you have a strong upper range (to the point where you can sing tenor parts) you should be able to manage.
Dickon is just right for your age though, so that shouldn't be a problem.