minecraft-assets/assets/minecraft/blockstates/nether_wart.json

{
  "variants": {
    "age=0": {
      "model": "minecraft:block/nether_wart_stage0"
    },
    "age=1": {
      "model": "minecraft:block/nether_wart_stage1"
    },
    "age=2": {
      "model": "minecraft:block/nether_wart_stage1"
    },
    "age=3": {
      "model": "minecraft:block/nether_wart_stage2"
    }
  }
}