Anyone find long games mentally tiring?

If games go on longer than say 30-40mins I struggle to maintain the same concentration I had to begin with and my playing becomes sloppy.

Is it because I'm approaching middle-age or does everyone else just consume Monster energy drinks to keep going on long games?

I think it is a mentally tiring game, especially if one tries to take on too much at once (manage multiple factories and striders, or try to help other lanes as well as your own etc).

Depends on the context of the game.
When i lose my morale im just demotivated, also when its at a current stalemate.
When i amit gives me more motivation.

At a certain point im not very focused on the game. (40-50 Minute benchmark.)
All advice i having a break once in a while when you get tired, personally when im well rested i can focus on the game more and be more productive.

When i want a more relaxing game, i play Coop,
also largely depends on my mood.

Without some examples I would be cautious to blame it on tiredness or something else. The longer time the game goes, some individual actions (like microing every glaive) will have a smaller impact - so you might not do it (unconsciously) because it is not efficient.

That being said you should choose to focus on something and that in itself can become quite tiring as there are so many things you could focus on (similar to "choice overload"),

I also find it that best players are the ones that decide (instinctively) on what to focus late game. If you watch replays they do not do 1000 things at a time, but rather couple of things and well. I feel more relaxed when I am confident I am focusing on the right thing than when people give me armies across the map because they resign - thankfully this does not happen very often due to my rank. I really dislike that and I try to rarely resign as it can be very frustrating for the receiving player.

I also get tired out and I think thats pretty normal... It`s not just the focus, think of how many things are going on in a clusterfuck and how many informations your brain needs to process. It does have to do with age as well, but it is also a matter of practise. Don`t fall for that "I am too old to do x" thing :)