So, if you’re thinking that a Cornish Game hen is not a Cornish Game hen at all, but rather a baby Cornish Rock Cross, you’d be right. Cornish Game hens are not raised commercially any longer because they take too long to grow to a marketable weight.
http://www.suburbanhomesteading.com/the-great-chicken-debate/barn
The first time I raised cornish rock cross, I was not prepared for ... how different it is to raise them. By harvest time I had a 30% mortality rate! I was sure I was somehow inadvertantly torturing these birds! After doing a lot of checking around, I found out that most people experience at 30% mortality rate.
https://permies.com/t/2006/critters/making-raising-cornish-rock-cross
There are people that raise cornish-rock-cross and get a mortality rate under 5%. I have been able to get it down to 15% and have a lot of ideas on getting it in line with those that get less than 5%. The first thing I've done is to never keep more than 25 cornish-rock-cross chickens in a pen (paddock) at a time.
https://permies.com/t/2006/critters/making-raising-cornish-rock-cross
Cornish Game Hens. True Cornish game hens are young female Cornish chickens weighing about 2 pounds. Grocery-store Cornish game hens are rock Cornish cross hens of the same size. They are butchered at 4 to 5 weeks of age. A poussin, or spring chicken, is a 3- or 4-week-old chicken weighing 16 to 24 ounces. They are very tender and popular in Europe.
http://animals.mom.me/difference-between-chicken-cornish-hen-5812.html
In a large bowl, mix soy sauce, oyster sauce, garlic, and pepper. Place Cornish game hens in the mixture, and marinate 10 minutes. Transfer hens to a medium baking dish, and cover with the marinade mixture. Cover dish, and bake 30 minutes in the preheated oven. Remove cover from dish, and place 1/2 the butter on each hen. Increase oven temperature to 350 degrees F (175 degrees C).
http://allrecipes.com/recipe/40453/seasoned-cornish-hens/
How to Raise Cornish Cross Chickens. People raise Cornish cross chickens or broilers for meat. They're fast-growing birds which mature to eating weight by 8 to 12 weeks. Hatcheries develop Cornish cross hybrids from white Cornish chickens and white Plymouth rock birds.
http://animals.mom.me/raise-cornish-cross-chickens-2179.html
Cornish Chicken. The Cornish, first known as the Indian Game chicken, was developed around 1820 by Sir Walter Raleigh Gilbert of England. General Gilbert claimed to have produced the breed from a cross of Red Aseel over a Black Breasted Red Game of the Lord Derby type.
http://livestockconservancy.org/index.php/heritage/internal/cornish
A 18% chick grower should be fed to the chicks from 5 weeks until butcher time. The Cornish cross are typically butchered between 8-9 weeks of age based on your preference or the weight of the bird. At 9 weeks of age, males will be around 10 pounds, females around 8. Cornish Game hens are the same bird, but butchered around 4 weeks of age, or around 2.5 pounds. Despite the name hen, these can be male or female.
https://www.purelypoultry.com/blog/raising-cornish-cross-chickens-for-meat/
Raising Cornish X for Meat Production. This year I am going back to the Cornish X for my meat chickens. I can expect to have birds dressing out to around 5 or 6 pounds each at about 8 weeks of age. This will allow me to grow out 25 or so chickens and have them in the freezer before the weather heats up too much for the young birds.
http://www.theselfsufficienthomeacre.com/2013/04/raising-meat-chickens.html
They are bred to grow the same, finish out the same and be similar in weight. This fast-growing breed is called a Cornish Rock Cross. Typically, they range in age from 4-weeks, for a Cornish Game Hen, to 8 to 10 weeks for a full grown roaster. The chickens are the same; only their name has been changed, taking labels given to chickens from days gone by.
http://www.suburbanhomesteading.com/the-great-chicken-debate/barn
how long to raise cornish game hen
 You can raise Cornish game hen upto 4 to 5 weeks of age. The Cornish game hen to raise are butchered at 4 to 5 weeks of age.