Without the specific averages, but using the DMG guide that a monster's AC goes up by 1 every level, and assuming the PC's get a +1 (wpn/impl) starting at 3, 8, 13, etc., and boost their primary attack stat at every opportunity, and started with an even stat (go away silly 17 or 19 builds, but 16 or 18 or 20 doesn't matter, you improve at the same RATE as everyone else even if you started behind)...
Between attributes, magic items, and half-level attack bonuses, you stay even except:
Level 5: Behind by 1
Level 11: Behind by 2 (total)
Level 19: Behind by 3 (total) (you can ignore this if you're taking Demigod, etc. at 21)
Level 25: Behind by 4 (3 with Demigod)
So Level 5 / 15 / 25 is a reasonable compromise, spllitting the difference between Level 11 and Level 19. If you do take Demigod (it's still really good), you actually come out slightly ahead of the curve, which is OK consider some crazy defenses and the fact that there are Level 33+ monsters (solos, at that) to be challenged.
Now if you want to actually take the averages, well, knock yourself out, but that's a lot of work unless you have it already in an analyzable form (see the Monster Stats thread).