Does this mean you don't think it likely/possible to make new friends as an adult?

I am unsure whether I think this, but I haven’t found new friends since I left school.

I am 25.

Personally, for most of my closest friends, I made them at around you age (I am 40).

The reason is simple: I moved when I was 23, to a place where I didn't know anyone. I met people there, we did stuff together, often, because I was very available, because I didn't yet have other people to spend time with, and now we are friends.

If you already have an established group of friends, you are probably less available to make new ones. But if you find yourself in a situation where distance is created, that's when you are the most available to make new friends.

I'll offer a counter-example, to give you a bit of hope. I'm in my 50s. In the last 5 years I've gained one close friend and a handful of new friends in my social circle. Age is no barrier to forming friendships. You must be intentional about it. But new friendship happens throughout our lives.

That's an honest and thoughtful answer :)

I'm only a decade older than you, but when I look back over that decade it feels like everything's changed. I've made new friends through hobbies, music, sports and yes, the workplace. I've lost touch with some people I thought were close friends. I've also been surprised by getting back in touch with people from school or university who I thought I might never hear from again.

Would I lie for those people in court (or vice versa)? I dunno. It's kind of a weird litmus test, because it depends on one's principles, how serious the charge is, and one's perspective on the legal system. It could happen, but I could only answer on a case by case basis. It's possible that I'd lie in court for a complete stranger if the circumstances were right. I struggle with absolutist views like "This is what friendship means and I will 100% behave in this way if that situation arises".

How would I define "friend" as opposed to "acquaintance"? Possibly as simple as: someone that you spend time with just for the enjoyment of it. If they're a work colleague, this means that you're likely to meet up socially, away from the office and work events (even if just online). Otherwise you're there because you have to be.

"Eg. I'd never lie in court for my acquaintances, but I have for my friends."

You have lied in court. Is this correct. How do we know you are telling us the truth.

Just assume everyone always lies on the internet

That’s the neat part: You don’t!

But: It’s not hard to say „no, I have never seen him doing drugs“ or whatever.

Don’t expect everyone to say the truth just because game theory suggests to optimize for individual rather than mutual reward.

My "best" friends that I am still in contact with are definitely from childhood. We all live in separate parts of the world now, but if I really needed help I know that they would come, just as I would go to help them.