If you have raised them up to laying eggs and past the screaming " was that a crow" stage you are well on your way. Most that die of heart attacks or have extreme leg problems die between 6-14 weeks. It sounds to me they are around 24-30 weeks old, which is great.
Some things to caution are feed, heat, water, and exersise.
1) When feeding don't let them eat a lot or whenever they please. Give them what they can consume in about an hours time 2-3 times / day. If not they will over eat and literally eat themselves to death. Make sure the protein in the feed is low...around 15%.
2) Heat...Like mentioned before this is one of the toughest things to deal with, do your best to keep them cool.
3) Lots of water never let them run out...there is a lot of meat on these birds so make sure they have plenty of water to go round.
4) Probably the best thing for these birds is to force them to exersise. By feeding them limited feed they will act like they are starving....don't fall for them....they are fine. They are pigs when it comes to food but with time they will learn to search for bugs, eat grass, dust, and scratch. They will search for food just fine and keep those joints in their legs healthy.
Broilers are genetically modified for growth, they are bred to gain weight fast. With this people have the assumption that all they do is eat and sleep. Which if left in contact with feed they will. The poor things will eat and as they gain weight they will eat to compensate for the amount of calories it takes to stay alive and what it takes to gain weight. Before they know it, it's a constant battle to meet their daily intake of food as day by day the amount of calories it takes to run these tanks increases. Before long it's too late and the amount of meat on the birds makes them uncomfortable to walk so they sit....right next to the feeder. They are now bored....can't walk....so they eat more.
The genetics in these birds are modified but the instincs haven't changed. When these birds have the right kind of exersise they will be fine. What we do here on our farm is when the birds get around 6-8 weeks we make them exersise by not feeding first thing in the morning. I go out around 6:30 a.m and prop the chicken tractors up off of the ground so the birds can go outside of the four walls. These little basketballs will range up to 100 yards away devouring anything that moves, they are hungry and their instincs tell them to look for food....not sit and wait for it to come to them. After about an hour I come back with a 2 wheeler of feed stacked on it and fill each pen......you should see 300 supersized cotton balls run....at 8 weeks weighing over 7 lbs ea. they look like Sumo Wrestlers on steroids!
GOOD LUCK don't let the statistics bring ya down.