Default value for missing properties with JSON.net

I found the answer, just need to add the following attribute as well:

[JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate)]

In your example:

class Cat
{
    public Cat(string name, int age)
    {
        Name = name;
        Age = age;
    }

    public string Name { get; private set; }

    [DefaultValue(5)]            
    [JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate)]
    public int Age { get; private set; }
}

static void Main(string[] args)
{
    string json = "{\"name\":\"mmmm\"}";

    Cat cat = JsonConvert.DeserializeObject<Cat>(json);

    Console.WriteLine("{0} {1}", cat.Name, cat.Age);
}

See Json.Net Reference

Add [JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate)] ,Change your Age property from

    [DefaultValue(5)]            
    public int Age { get; private set; }

to

    [JsonProperty(DefaultValueHandling = DefaultValueHandling.Populate)]
    [DefaultValue(5)]
    public string Age { get; private set; }

You can also have a default value as:

class Cat
{           
    public string Name { get; set; }
        
    public int Age { get; set; } = 1 ; // one is the default value. If json property does not exist when deserializing the value will be one. 
}