Where can I find a list of some sort...

[Buczilla]

for everything possible to display on viewtopic...

ex.
Age, Usergroup (not ID), etc.

[Gary King]

So you want to display the age, usergroup, etc. of a member in the postbit?

For usergroup, placing $bbuserinfo['usergroup'] somewhere in the postbit will display the usergroup of each user in their postbit.

For the user's age, try this hack:
https://vborg.vbsupport.ru/showthread.php?s=&threadid=42050