I tried to stick with second year shoots this fall, but found it's actually very difficult, at least in my local patch of thorns... second-year shoots are way too large in diameter for arrows, they drop down and get bendy with all the weigh of new side-branches, and the side-branches leave big knots where the wood splits and makes straightening really hard.
I harvested my first batch of rose shafts last winter without realizing the first/second year thing, and still got some nice arrows. This fall I harvested again, all this year's shoots. I cut them 1/2" thick, because once they shrink and the bark is off, they lose alot of diameter. It seems like you need to cut them fresh alot thicker than you'd think, so I actually take a set of cheap calipers with me to be sure. Half the shoots I was bringing home were way too skinny once I started sorting!
My best arrow is 3/8" at the nock and 5/16" at the tip, 27" long.