You could use Between.
AgeGroup:Iif([AgeField] Between 0 And 17, "0-17")
There is another option - create a table with all possible ages and corresponding age groups.
Something like this perhaps.
<table bgcolor="#ffffff" border="1" cellspacing="0"><caption>
</caption><thead><tr><th bordercolor="#000000" bgcolor="#c0c0c0">AgeGroup</th><th bordercolor="#000000" bgcolor="#c0c0c0">Age</th></tr></thead><tbody><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">0</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">1</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">2</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">3</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">4</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">5</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">6</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">7</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">8</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">9</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">10</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">11</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">12</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">13</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">14</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">15</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">16</td></tr><tr valign="top"><td bordercolor="#c0c0c0">0-17</td><td bordercolor="#c0c0c0" align="right">17</td></tr><tr valign="top"><td bordercolor="#c0c0c0">18-35</td><td bordercolor="#c0c0c0" align="right">18</td></tr><tr valign="top"><td bordercolor="#c0c0c0">18-35</td><td bordercolor="#c0c0c0" align="right">19</td></tr><tr valign="top"><td bordercolor="#c0c0c0">18-35</td><td bordercolor="#c0c0c0" align="right">20</td></tr><tr valign="top"><td bordercolor="#c0c0c0">18-35</td><td bordercolor="#c0c0c0" align="right">21</td></tr><tr valign="top"><td bordercolor="#c0c0c0">18-35</td><td bordercolor="#c0c0c0" align="right">22</td></tr></tbody><tfoot></tfoot></table>
This can then be used in 2 ways.
1 Add the table in a query and link the age fields.
2 Use DLookup, eg AgeGroup: DLookUp("[AgeGroup]","AgeGroups","[Age]=" & [CustAge])
SELECT Customers.CustName, Customers.CustAge, DLookUp("[AgeGroup]","AgeGroups","[Age]=" & [CustAge]) AS AgeGroup
FROM Customers;