Return Number of Unique Valued Based On Specific Text

I am trying to create a formula that will return the number of unique items in column B, based on what column A says.
Example:
I have tab titled "June" and column A titled "PO Open" and Column B titled "PO #". I also have a tab titled "ChartData" that will be used for building a graph.
I would like cell G3 on the ChartData tab to tell me how many unique items are in column B June tab when column A says 12-18Mo.
I would like cell G4 on the ChartData tab to tell me how many unique items are in column B June tab when column A says 18+.

I know I can do a PivotTable and then manually enter that number onto the Chart Data, but I was hoping there would be a way to create a formula so it would automatically pull in the count of unique POs.

I have attached the file.

Thank you for any suggestions in advance.

PERFECT!! Thanks for the assistance