OK so...

Im in the cube. I go into the tab dimension useage

for the Age dimension the measure group column side is underlined in red

The Key column has data type Wchar different that the attributes ID of Integer. To correct this issue edit the relationship or delete and recreate it.

OK so I go in to try and delete and I cant see anything about the key column, Even when I go into advanced.

I could try and create the relationship again in here but I'm guessing I'm still not in the right area that you are talking about