You need a good fan.

Pare your kit down to nothin'

Take your Geritol and don't booze.  Motrin is your friend the next day 

Playing with economy of motion helps (no fun though) but 100 degrees is a 100 degrees-  my friend was a florida drummer for a long time and Mr. A is also.  Only take air conditioned gigs?

Rockin' is alot different now than in your 20's ain't it?  I had a sweat fest this past weekend also- got thru it alright and played pretty good.  I'm not a big sweat head usually, but at least a couple of times a summer, when I least expect it, I'm in a hot place.  We had A/C but it wasn't anything to write home about.  I didn't get my core temp down until I froze myself in the van on the way home- it took at least 20 or 30 minutes.

I'm 47, live in Florida, and play plenty hard. The secret is pacing yourself, and making things as easy for yourself as possible, so you can focus your energy on playing. Use a fan, use the simplest, lightest, easiest-to-pack-up gear that can still get the job done, use the most comfortable throne you can afford, possibly with a backrest, make sure you've eaten enough and aren't drinking too much booze, and drink plenty of water. If caffeine helps, load up on coffee, diet coke, or mountain dew. And when you take breaks, take advantage of them - stand up, get away from the kit, and let your body relax before the next onslaught.

But pacing yourself is probably the most important. Be aware of how much of the gig is still ahead of you, and hold back a little if you need to make sure there'll still be enough energy left for that final set.

I'm spoiled in that recently I've mostly done single or double sets, not the 4,5, or 6-set marathons I used to play. When I do play a 3- or 4-set gig, I'm pretty tired by the end. But I also pace myself carefully, not drinking too much beer, and keeping an eye on the clock. There's nothing worse than fading out before the gig is done!

I have found caffeine makes me too tense and things don't flow smoothly when I play. But it might work for other people. It's interesting, there is this killer drummer north of here who plays professionally, gives lessons during the week and travels too. I went to see the band and there he was packing up a little practice pad kit that he works out on an hour before each gig so he's very warm. He said he used to just do the pad for the hands but then found his feet were'nt as warmed up as his hands so added the bass drum pad. I was surprised as he is so accomplished and has been playing over 30 something years. He's in pretty good shape and doesn't smoke. I did see him eating cookies. I think drums are so physically demanding that it does make sense to warm up like an athlete. This warm up he does is in addition to regular practice. He's phenomenal so it's working for him. He's in demand and he makes his living as a drummer. 

44 is definitely not too old. E.g., Steve Smith will be 51 in August, and going stronger than ever... so are countless other older players.

I suspect you just had a down night... no big deal.

In addition to the comments / suggestions above, good technique can help make things a little easier. E.g., staying relaxed in our playing and not hitting harder than we need to helps conserve energy.

I am 54 - today as a matter of fact, and I truly don't feel any effects which would be caused by fatigue. I still play plenty hard but we organize our sets so the order of the songs dosen't burn anyone out. I also exercise regularly with weights and areobic types of activities. I really believe that is the key. Heat exhaustion is another matter. As Dave said, it is VERY important to be aware of your body's need for fluids. So drink lots of water and keep on rockin'!!
mp

I'm 58, and here in Texas am constantly playing either outside or in venues whose air conditioning is less than optimal.  A few weeks ago I played a 2 1/2 hour big band tuxedo gig (with no breaks) in the afternoon, followed by a 4 hour rock club gig that night.  I moved, set up, and tore down all my gear by myself for both gigs.  By the time I got home I was pretty whipped, but the gigs themselves were surprisingly easy.

All the above suggestions about dealing with heat, like water, fans, etc., are right on, and I follow them religiously.  I don't drink booze until I'm through playing, and I don't go to gigs hungry.

For a time last summer I was in a trio that played very energetic jazz at an outdoor gig every week in July and August.  Temps were routinely over 100, peaking around 105-107.  We had 3-4 large fans going all the time, and I have a small one behind me now for just about every gig.

44's only too old if you're not in shape.  I know two drummers 75 and 82 who are still playing regularly, and the 75 year old plays 3-4 hours 3 nights a week, often in a suit.  The 82 year old plays in 3 bands that I know of, and called me once to sub when he had simultaneious gigs booked.  They're my idols, along with 80 year old Roy Haynes.

When I know I'm working in hot weather I load up on carbohydrates and salads all week. (pasta...lots of it)....Then on gig day I stick to water or Gatorade....By the way, I'm 58. Worked last week at a motocross national as a checkered flag man in 100 degree heat. Got back to 100 degree heat in Minnesota. I've got 3 gigs this week and pumping starch again.... Don't let the band push you too hard. Tell them you need to pace yourself. It's different for drummers....The heat doesn't bother me. I had a roofing business for over 25 years plus played 5 nights a week, and raced on weekends. The carbs and water was the only way I survived those years.... Pace yourself and have fun. Booze will bring you to a halt in hot weather.   

Actually I'm not sure about the caffiene, but anything with sugar has a less than optimal effect on hydration.  It takes 1 molecule H₂0 to break down 1 molecule sugar.  A percentage of any liquid consumed with sugar in it goes to the digesation of it.  I'm not sure how it works with diet drinks, Aspartame, Nutra sweet.

Another dinosaur (53) checking in and just got home from playing a big band gig with plenty of energy in over 90 degree heat. Went through a couple bottles of water each set and a couple during the break. Feeling great and helped the other guys load the pa gear too. I'm by no means in great physical shape but I do pay attention to what I do and how I lift things. I'm in complete agreement with keeping well hydrated with water as being a key to feeling fit.

I am 48 and still playing those type of gigs also.  I have actually started bringing my own mic's and board and speakers to the gig because I am involved in some rather loud bands.  I just have one question that I have been asking for about 20 years now.  WHERE'S THE ROADIE??  Wouldn't it be nice to just show up and have the kit in place, play the gig, have some fun and then just split. Next night . . . . same thing.