Try
<display:table name="clients" id="client" uid="row">
<display:column sortable="true" property="id" title="ID"/>
<display:column sortable="true" property="name" title="Name"/>
<display:column sortable="true" property="surname" title="Surname"/>
<display:column sortable="true" property="age" title="Age"/>
<display:column sortable="true" property="email" title="e-mail"/>
<display:column sortable="true" property="sex" title="Sex"/>
<display:column>
<s:form action="delete" method="GET">
<s:hidden key="client[%{#attr.row_rowNum - 1}]" value="%{#attr.row.id}"></s:hidden>
<s:submit value="Delete"/>
</s:form>
</display:column>
</display:table>