vacuum tx id wraparound issues

I'm seeing a number of vacuum's in one of our db's that has a notation (to prevent wraparound).

I've tried a number of things to fix it. last nite during the off hours we ran a "vacuumdb -a"

The vacuumdb did vacuum every db in the cluster but I'm still seeing the vacuums to prevent wraparound.

a few days ago this query showed an avg of 200million for the age, now its 100million.

postgres=# select datname, age(datfrozenxid) from pg_database;
datname | age
-----------+-----------
template1 | 36020008
template0 | 36017769
postgres | 101264283
report | 101264283
kabc | 100009842
prod | 101264283
quota | 101264283
cfg | 101264283
(8 rows)

why did the vacuumdb -a not clear this up for us?

Thanks in advance