When the temp is below 80F I can roll with the big boys but when it is above 85 I am wrung out in no time. I know I am in trouble when my legs start to feel cold while the sweat is pouring off of me. For background I am 55 years old. What is happening? What can I do?
What you describe is not really heat exhaustion per se. Heat exhaustion is weakness associated with dehydration. Sun Stroke is the next step. Sun stroke is severe dehydration to the point where sweating stops and the body temperature rises. Sun stroke, not too surprisingly is life threatening.
I have a similar problem. Here in the summer in Central FL, I have trouble on hard rides on hot days (most summer days). It seems as if my body trys to vent heat by sweating more but all it does is roll off of me and not cool me but it does dehydrate me. I lost 20 lbs last year (I am 6' and 180) and losing the insulation seems to have helped. But I still drink more than most people and sweat far more.
In your case, it is probably genetic. Getting really lean would probably help but that can be a daunting task. You might try drinking more and seeing if you can keep up with the sweating. On long rides (over 50 miles) you might suffer from electrolyte imbalance also.
I have a lot of experience with dealing with heat and heat exhaustion, having lived in Sahelian West Africa for three years. I suffered from heat exhaustion twice, and your description sounds more like early dehydration.
First thing to realize: while genetics may be important, the human body is designed to be active in extreme heat. We are one of the few species of mammals that can do that, camels come to mind as another, and they're not particularly active.
Once you understand this, then you need to figure out how to maximize this inherent trait. Our cooling system--sweating--is unique, and very effective. In order to maximize the process, however, you need to be able to sweat well and efficiently. One way to do this is to sweat as much as possible. Be active in heat and keep yourself sweating. You don't have to go hard riding during the day, esp. if you get worn out, but don't go running into an a/c environment either: instead, keep drinking water and letting yourself sweat. Get used to feeling the heat and try and become comfortable sweaty. The other thing to make sure to do is to increase your salt intake. The more you sweat, the more salts you use up. Those need to be replaced, along with your fluids. Gatorade works for this. If you don't have gatorade, drink something with sugar, and add a teaspoon of salt to it. (Aside: key to great homemade lemonade made with fresh lemons is salt, nature's flavor enhancer). Also, increase your salt intake during your meals. Not a lot, but if you're exercising and sweating a lot in the heat, and you're craving salt, then eat more of it.
Final note: you'll know heat exhaustion when you feel clammy and you stop sweating. Should this occur, and you drink some water, you'll be amazed to begin sweating within seconds of drinking. Other symptoms of heat exhaustion are extreme headaches, dizziness and utter, poop-tired exhaustion. Should this happen to you, you must get yourself to a cooler environment, lie down, and probably sleep. You cannot rectify the situation by continuing to be active and just trying to 'catch up'. By the time you've got to the heat exhaustion stage, your body's entered into minor trauma. It's similar to altitude sickness: once you start having symptoms, you have to stop and let your body recover before pushing it further.
Good luck, and really enjoy the heat, it's a great lubricant for the joints!