using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        string data = "{\"_age\":3}";

        var testA = JsonConvert.DeserializeObject<TestA>(data);

        Console.WriteLine(testA.Age);

    }

}

​

public class TestA

{

        [JsonProperty("_age")]

        public int Age { get; set; }

}