I don't suppose there is a list of AA available by level?
I've never played with the in game aa filter but have you tried there to filter by level?
I do not believe there is a by level filter. I suppose you could go by expansion, but that will not work for everything.
For instance-there are 33 levels of the defensive abilities.
3 available at 55. Leaving 30. They start to become available again at 61. But there are 25 levels after 60, and 30 levels of defensive AA. So somewhere, there is a point to buy those extra 5 levels. Meaning you might be a bit more capable defensively, relative to the mobs at that point then at anytime else. Where is that? But since the first level of those AA are available at level 55, you can't figure that out from sorting by expansion.
I have see level-based lists in the past, but I don't know of any that were updated after the last major AA-revamps, which combined some lines and moved them around a bit. You'd probably just get confused looking at one if you didn't know the historical arrangement of AA's.
One way to get a quick level filter in-game would be to have 12 or more unspent AA's (nothing costs more than 12). Click on the "Can Purchase" button, and you'll see what's available to you currently. Anything above your level will not be listed.
That doesn't help with looking ahead though.
I'm particularly interested in when you can 'double up' on defensive AAs. Between 61 and 85 there are 30 levels of Combat Agility/Combat Stability. So somewhere in there, there is a range where you can get get more than one set of defensive AA per level. Where? That's my big question. Everything else is just an annoyance, really.
Omens of War and Depths of Darkhollow both had defensive AAs available and were level 70 cap expansions.
They both had 5 ranks each, I believe the OOW set went 1 rank per level 66-70 and the DoDH ranks (which came later) each required level 70.
To answer your question level 70 is the sweet spot for defensive power. There were also some really awesome AA spots at level 70 (WoS Murks and PoFire C1 and C2) but it's been so long they might not be relevant with hotzones and mercs.
I created a fairly accurate chart of our AA. I have linked it in my sig. The chart is complete as far as I know but doesn't contain AA by level.