SSRS Chart grouped by range of values

Hello,

Can you guys assist in the following scenario where I have two columns  "Staff"  and  "Age"

Staff     Age

John      23

Bob       25

Mike      27

Caren   35

Bonny   42

I'm trying to build a Column chart in SSRS 2008 that would Count th number of staff in every age range (20's , 30's, 40's)

I'm hoping to get three columns in the chart : one field for the 20's  with 3 people in it;    and two other columns for the 30's and 40's with one staff in each.

I can't figure out the expression that should be used in the categories group of the chart.

thanks in advance