anything over 8 years old.I've had cubans from the late 80's to this year. Some are good young,some after 4 years,and so on. However,every stick I have had with 7 to 8 years on it has been epic. The exception to this is ,as was mentioned,years between '99 to mid '01,when rapid growth and poor business practices made cuban cigars a crapshoot. Poor rolling and the rush to get product out is why I avoid buying these years,and why you see a lot of '01 stuff in "Vintage" sections on web sites. However,a good stick from this period can be amazing.
2005 is widely considered a good year,I have found it to be so myself. The 2008 to present have been very good in my experience,with emphasis on 2010 and 2011. Excellent young sticks,probably will go thru a "dumb" phase for a few months,and hopefully mature into their typical cuban profile.
I have had more good surprises among the less popular marcas,and generally prefer the 46 and smaller guage sticks. Also, I have found that Cohiba needs 4 to 5 years to really start showing,and hit their stride at 7-8 years.