Doctrine_Core::ATTR_IDXNAME_FORMAT can be used for changing the naming convention of indexes. By default Doctrine uses the format [name]_idx. So defining an index called 'ageindex' will actually be converted into 'ageindex_idx'.
You can change the index naming convention with the following code:
// bootstrap.php
// ...
$manager->setAttribute(Doctrine_Core::ATTR_IDXNAME_FORMAT, '%s_index');