Remember, up till September/October this year everyone was still using 2014 hops. And many still may.
From what I gathered, most warehouses and vendors store them cold, but AFAIK not frozen. Deep frozen is the best to retain freshness, aroma, and bitterness over time as chemical processes and breakdown slows down with lower temps. If you're buying 2013 harvest hops now, chances are they will be fine but may have lost some character over 2 years. I think you can easily afford to use a bit more.
I still have quite a few 2012 harvest hops left, and thinking about it, some maybe even from 2011 since there's no date on them. I bought those in 2013 (HopsDirect) and they've been deep frozen (-4F) since I got them. They're all pellets and still smell and taste as good as the first time I used them. There's no yellowing or browning anywhere to be found. I have no doubts when using them now.
In recipes, after adjusting for %AA, I add an additional 10% of weight for each year of age, and that off the cuff formula seems to work fine. I haven't had any issues with lack of flavor, aroma, or bittering.
Go for it, there are some great deals to be had!