when should i do ancients

[Crystalion]

The minimum levels for full experience seem to be 36/66/74.

Another issue (not addressed in the AB thread last I looked) if you want to "rush" past the Ancients as early as reasonable is that if you are clvl 20 to 23 then you will still be below ("magic" clvl) 25 when you get your Ancient's level up bonus. This means that everything past the Ancients, being, iirc, at least level 36, will be getting massive xp dilution when you kill them.

If you wait until at least clvl 24 to do the Ancients you will emerge clvl 25 and the xp anti-rush nerf will be off (that is, you can fight for xp anywhere in the game from a little below your clvl to mlvl 110 without fear of harsh xp dilution). Prior to clvl 25 the "too far above you" xp nerf can take you down to as little as 2% of the xp you "should" get.

If you do the Ancients at clvl 24 instead of 36 you do give up quite a bit of Ancient's xp bonus (due to their one level up cap)... several levels worth of xp (in clvl 24 terms, not clvl 36 terms), iirc. This is not necessarily a big deal. For the nightmare and hell rewards the situation is probably much more unpleasant doing the Ancients "early" (not sure, as I've not run the numbers for those myself). OTOH, given the "guest" monsters in nm/hell act 5 one might have a much stronger wish to "get the hell out" of act 5 sooner and on to the next thing (act 1 hell if coming from nm, act 1 cows if coming from hell). In particular, if you have a build that fights very well above your level vs. undead it might make some sense to exit act 5 nm (and, presumably, the zombie garden) ignoring Ancient's bonus issues, so that you can do the special Mausoleum/Crypt areas in act 1 hell (mlvls in the low 80s, iirc--see Ruvanal posts on topic).

Of course rather than considering all this tech. stuff why not just do the Ancients when you *can* when you feel like it? There isn't much point to "saving time" gaining xp if you're not enjoying what you're doing in the first place. YMMV.