Remathilis
Legend
You could play as a Barbarian or a Monk and have it by level 8 (or possibly even 4), if you wanted to.
Having plate unavailable at or after that level would be unnecessarily punitive to the classes that could use it, unless you compensate in some other fashion.
Sure, you could devote your two highest scores* (15, 14) to Dex and Con/Wis, pick a race that grants boosts to them (hill dwarf, human, half-elf, etc) and then devote both your stat bumps to it, then sure, you can have two 18s and have an 18 AC. However, that's a huge amount of resources, as well as the cost of not raising your other scores (such as Str for the bbn) or picking any feats until 12th level.
To be honest, slapping 1500 gp down sounds a lot easier to get an 18 than that!
Doable =/= easy. There are lots of Doable ways to get an 18+ AC. None of them are easy. They require high scores, feats, class features, or lots of GP (or magic). And almost all of them cost you in the offensive category in some way. Seems a fair trade.
I just despise this idea that having plate as rare is "nerfing fighters" I got sick of equipment as players right two editions ago...
* Standard Array. Customized scores could get you a little closer, and rolled scores are "all bets off"