I would suggest you to have three measures as below
[Age <4]=Count([Number Of Incidents] Where ([Open Incidents]<4);All) In ([Country])
[Age 4-9]=Count([Number Of Incidents] Where ([Open Incidents]>=4 and [Open Incidents]<9);All) In ([Country])
[Age >9]=Count([Number Of Incidents] Where ([Open Incidents]>9);All) In ([Country])
and use them in your table.
Regards
Niraj