From Runners Health:

"It is typically more difficult for older adults than for younger people to keep their core temperature within a normal range, which makes them more susceptible to heat related stress and illness.

A study published in PloSONE by Joanie Larose and colleagues showed that this decline in thermoregulation starts already as you reach 40, and worsens gradually as you get older. This is important if you are a master athlete planning his/her race calendar for 2014: you might want to plan your most important races when the weather is not too warm.

Joanie Larose and her colleagues examined 85 male volunteers between 20 and 70 years old during 15-min bouts of cycling separated by 15 min rest. The total duration of the test was two hours.  To calculate the amount of heat generated, the researchers measured the volume of O2 and CO2 expired, as these volumes correspond to the amount of fat and glycogen burned to obtain energy. As they knew from the cyclometer how much of that energy was used for exercise, they could easily calculate how much had been transformed into heat.

The researchers used direct calorimetry to measure the total heat loss. This means that they examined the air of the cylinder-like room in which the volunteer was cycling, as any change in the temperature and humidity was due to heat loss from the cyclist.  The difference between the heat generated and the heat dissipated is stored in the body, and results in thermal strain.

Joanie Larose showed that from 40 years onwards, men gradually become less good at dissipating the excess heat.  This is important, since it could lead to an increased core temperature.

As this is dangerous for your health, your unconscious brain will do what it can to keep your core temperature within the normal range.  During exercise, it will therefore slow you down by making you feel tired and reducing the amount of muscle fibres you can use.

Consequently, master athletes will find it more difficult to compete at their best in warm conditions. It is thus a good idea to plan your A races during cooler periods of the year.

This study has been performed on men, and we cannot assume that the same happens to women. We just do not know. Most studies show that women tend to sweat less than men, but it is no clear yet how this affects their thermoregulation.  We need more research."

(link)

The effect would likely diminish but not go away.  You can't undo the damage that time does to your body and this is one more example.  Moreover, even if you are young and acclimated, you can't run as fast in hot weather as you can in cool weather.

We frequently talk about VO2max, lactate threshold, muscular endurance, etc as limiting factors on your ability to run fast but we rarely talk about your body's ability to dissipate heat as a limiting factor yet is most assuredly is.  I have watched the results from people who tried to run a marathon at goal pace on a humid day.  It's ugly.  Saw the same thing in a half marathon except that blowing up from overheating in a HM only means you have to walk 2-3 miles whereas doing it in a marathon makes you walk or jog 6-8 miles.

Evidence that as we get older, we get hotter.

 

This in particular is interesting - I don't sweat nearly as much as my husband.   In fact, when I was seeing doctors for the syncope issue earlier this year, my ability to sweat and cool myself came up as an issue - I get really flushed when I workout in warm temps, and the doctor said that's the body trying to overcome my inability to sweat enough to cool myself (so, blood rushing to the surface to cool me might have been contributing to the almost passing out.)  Turns out I still have the syncope issue in cold weather, but now I know I am crappy at sweating lol

When my DD tried to take up running  a few years ago, she could not sweat at all. She would run 4-5 miles and come home all flushed and dizzy and feeling like she was going to pass out. Not a drop of sweat on her. But one day, a few months later, she called me to tell me the good news: she had started sweating, finally, and was feeling so much better! Yeah for gross sweat! 

Interesting study, but at 58, I haven' t found it to be the case for myself - I still thrive in the heat like I always have, and freeze in the cold. When I was young I tolerated both heat and cold, but the older I get the less I want to deal with cold weather or running in the cold, because it just plain sucks to get cold.  I don't feel the same way about hot weather. I suspect the majority of the subjects in the study were probably sedentary and were showing signs of poor circulation and poor sweating/cooling ability caused more by lack of use than by aging.

My first running career (early 80's) was in Tampa.  I would regularly do 6 to 8 mile noon time runs - and while the heat was a factor, I think getting all that training in during the heat of the day helped me adjust to racing at elevated temperatures.  My 5k PR was set at an event called "beat the heat" - a race series that was run at 6:00 PM during the summer months.

Now (at a slightly elevated age) my summer noon time runs in the mountains of NC are a struggle and I look forward to cooler days or rainy days.  At the same time, extremes in temperature on the down side are equally as undesirable from a comfort level, but much preferable from a racing standpoint.

I used to go through spin classes and not break a sweat. 

That is really bizarre.  Meanwhile I sweat a decent amount running easy in 25 degree temps wearing just a singlet, arm sleeves and shorts!

As I get older, I get colder.  Now somebody get me a blanket and GET OFF MY DAMN LAWN!