Vlookup / Min Array question

welcome to the forum, katharineeileen. you are getting zeros because your years are in text forms. delete your array formulas first because i think you ranged the whole thing and pressed CTRL + SHIFT + ENTER. now use this array formula just for cell B2.
=MIN(IF(Lookup_ID=A2,--Grad_Year))
...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer.

then copy it down.

seeing you have 50K+ rows, it is a little slow though. you might want to consider using PivotTable and show the MIN Grad Year instead

@ benishiryo's I believe that can be entered with just Enter.

@FlameRetired: Unfortunately, you have to use the array formula. the IF formula alone is typically for single cell usage. you cannot use IF(A1:A10="x". basing on OP's eg, it all goes wrong at row 75 onwards once you do not press CTRL + SHIFT + ENTER.

@katharineeileen: provided that the underlying value of the cell is indeed a number (like in your case where year is merely formatted as a text), the double negations help to convert text formats into numbers. some people prefer these which works too:
=MIN(IF(Lookup_ID=A2,Grad_Year+0))
=MIN(IF(Lookup_ID=A2,Grad_Year*1))
=MIN(IF(Lookup_ID=A2,VALUE(Grad_Year)))

Originally Posted by benishiryo

@FlameRetired: Unfortunately, you have to use the array formula. the IF formula alone is typically for single cell usage. you cannot use IF(A1:A10="x". basing on OP's eg, it all goes wrong at row 75 onwards once you do not press CTRL + SHIFT + ENTER.

Yup I just noticed that too.