First see the code in global.php after this line:
if (defined("GET_CACHES")) {
Add the new field to the sql query. All category data will be saved to the $cat_cache array. From this you can register the value to the template engine.
For example in categories.php
Add after
"cat_hits" => $cat_cache[$cat_id]['cat_hits'],
this
"cat_age" => $cat_cache[$cat_id]['cat_age'],
Now you can use {cat_age} in the categories.html template.
Jan