Conditional formatting with lookup formula?

So as long as the last year is in 2011 or later, it's valid?

Perhaps try formula:

=IF($A2="","",IF(AND($I2=MAX(IF($A$2:$A$100=$A2,$I$2:$I$100)),YEAR(MAX(IF($A$2:$A$100=$A2,$I$2:$I$100)))>=2011),"Valid",IF(YEAR(MAX(IF($A$2:$A$100=$A2,$I$2:$I$100)))<2011,"Expired","")))

adjust ranges to suit... I guessed you are looking at column I for dates.. and also only went to row 100..

Then confirm the formula with cTRL+SHIFT+ENTER not just ENTER and copy down.

You can then set conditional formatting to see if Cell Value is ="Valid", turn Green, and one for if Cell Value is ="Expired", turn Red.