age(datfrozenxid) negative for template0. Is this normal?

Today I executed the following query as stated into the Administrator
Guide to check the XID wraparound problem:

SELECT datname, age(datfrozenxid) FROM pg_database;

All the database report an age of 1 billion except for the template0
database. This is an extract of the result of the query:

         datname         |    age
-------------------------+------------
 template1               | 1073794149
 template0               | -686262347

Is it normal that the age for template0 is negative?

The version of the backend is 7.4.6 with pg_autovacuum running.

Please let me know as soon as possible if this is a problem or not...

Thank you,
Denis