Alternatively, you can make your single select question have age ranges in the text labels, but record values 1,2,3.
In KoBoForm, that would look like this:
The XML ends up looking like this:
<h:body>
<select1 bind="A01">
<label ref="jr:itext('A01')"/>
<item>
<label ref="jr:itext('A01_1')"/>
<value>1</value>
</item>
<item>
<label ref="jr:itext('A01_2')"/>
<value>2</value>
</item>
<item>
<label ref="jr:itext('A01_3')"/>
<value>3</value>
</item>
</select1>
</h:body>
In the final form in KoBoCollect, the user will be presented with labels showing age ranges, and they can select a range. The values 1,2,3 will be recorded in the database. The only disadvantage here is that you will not know the user's EXACT age, you will only know the range. Otherwise, this is a simple and easy solution.