(US Based Answer) - Talk to your GP and they can likely recommend a local specialist who does the testing.

What was the value that you saw in getting it diagnosed? I'm sure many people live their whole life without having a label for some of the stuff going on in their body/mind. If they're halfway through life and not obviously suffering, is it worthwhile?

If you actually have ADHD, trying a stimulant medication will tell you in 15 minutes how quiet everything is for people without ADHD.

You can get a treatment if you’re diagnosed with ADHD. Being halfway through life doesn’t mean you should just idly wait for death.

I think you misunderstand me. Somebody halfway through life isn't just sitting around waiting to die. But if there are ADHD indications and no obvious suffering or underperformance, it is clearly a condition that has been lived with. How valuable is the treatment at that point? There's quite a range between "meh, if it hasn't been a problem then it's probably not going to move the needle" and "it is lifechanging, you don't know what you're missing.". Sibling reply suggested the latter certainly _can_ happen. It seems like a number of posters in this thread have personal experience/wisdom to share.

So, I'm not diagnosed (though I recently had a member of my care team recommend going through an evaluation to maybe shed some light on things and am starting that process). Truth the "ask your GP" is generally the answer to any question on how to start a process in the US medical system.