// step 3: convert string ages to floats
.Append(mlContext.Transforms.Conversion.ConvertType(
    "Age",
    outputKind: DataKind.Single
))

// step 4: replace missing age values with the mean age
.Append(mlContext.Transforms.ReplaceMissingValues(
    "Age",
    replacementKind: MissingValueReplacingEstimator.ColumnOptions.ReplacementMode.Mean))

// the rest of the code goes here...