In your blockstate json file for this block you have to state a model for each age it has.
{
"variants": {
"age=0": {"model": "MODID:REED_0"},
"age=1": {"model": "MODID:REED_1"},
"age=2": {"model": "MODID:REED_2"},
"age=3": {"model": "MODID:REED_3"},
"age=4": {"model": "MODID:REED_4"},
"age=5": {"model": "MODID:REED_5"},
}
}
add ages up to the ages you have set for your reed.
You can also refer to the same model for each age.
{
"variants": {
"age=0": {"model": "MODID:REED"},
"age=1": {"model": "MODID:REED"},
"age=2": {"model": "MODID:REED"},
"age=3": {"model": "MODID:REED"},
"age=4": {"model": "MODID:REED"},
"age=5": {"model": "MODID:REED"},
}
}