c# Generic data object using newtonsofts JsonConvert DeserializeObject

I would propose to consider you actually have 2 types of JSON response. You also can inherit them from base ApiResponse class:

using Newtonsoft.Json.Linq;
using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            string json1 = @"{
                                ""success"":true,
                                ""Data"":[{id:1, name:""Paul""},{id:2,name:""neville""},{id:3,name:""jason""}]
                            }";

            string json2 = @"{
                                ""success"":true,
                                ""Data"":{id:1,classSize:30,minAge:25, maxAge:65}
                             }";

            string j = json1; //json2;

            JObject jo = JObject.Parse(j);

            ApiResponse parsed;

            if (jo["Data"].Type == JTokenType.Array)
                parsed = jo.ToObject<ApiUsersResponse>();
            else
                parsed = jo.ToObject<ApiAgeResponse>();

            Console.ReadKey();
        }        
    }

    class User
    {
        public int id { get; set; }
        public string name { get; set; }
    }

    class AgeClass
    {
        public int id { get; set; }
        public int classSize { get; set; }
        public int minAge { get; set; }
        public int maxAge { get; set; }
    }

    class ApiResponse
    {
        public bool success { get; set; }
    }

    class ApiUsersResponse : ApiResponse
    {
        public List<User> Data { get; set; }
    }

    class ApiAgeResponse : ApiResponse
    {            
        public AgeClass Data { get; set; }
    }     
}

You could just make your class generic by adding generic type parameter for Data property:

public class ApiResponse<T>
{
    public bool success { get; set; }
    public T Data { get; set; }
}

And then use it when deserializing:

  var result = JsonConvert.DeserializeObject<ApiResponse<AgeClass>>(myJson ...);
  var result = JsonConvert.DeserializeObject<ApiResponse<List<User>>>(myJson ...);