If you have an artist where either (or both) of the start/end dates are not specific down to the date, then you can end up showing negative ages for the group.

Example:
Start: 1985-07
End: 1985
Dissolved: 1985 (-7 months)

Apparently, the server will no longer allow you to create an artist if the level of specificity for the end date is not at least as granular as the start date, if the year (or year+month, as applicable) is the same, following the above pattern. (bug?) However, there's plenty of these from pre-NGS. Example: http://musicbrainz.org/artist/6d797b95-2011-4859-91ab-9284d1e1f472 , who is showing a -2 month duration for the group.