Actually this seems to work for me, in spite of the red syntax coloring!
rdata = ResourceData["State of the Union Addresses"]
data = rdata[All, {"Date", "Text", "Age"}]
sample = IntegerPart[.9*Length[data]]
trainingDataset = RandomSample[data, sample];
testDataset = Complement[data, trainingDataset];
p = Predict[trainingDataset -> "Age"]
PredictorMeasurements[p, testDataset -> "Age", "ComparisonPlot"]

Or by transforming the Dataset:
tData = data[All, {#Date, #Text} -> #Age &] // Normal;
trainingData = RandomSample[tData, sample];
testData = Complement[tData, trainingData];
p = Predict[trainingData]
PredictorMeasurements[p, testData, "ComparisonPlot"]
From the same test dataset and predictor function as above, it gives the same result:
