Has anyone studied at the Hague Academy of International Law? What was the experience? Is it very serious for older professionals and/or phd students, or is it really geared towards early 20s students to hang out and make friends from around the world? I'm keen to find out more before committing time and money on this program.

The Hague Academy is a serious, traditional and prestigious institution.
I took the advanced course on international criminal law, and it was excellent. Most people were professionals, not students. Judges, prosecutors, senior lawyers, Interpol personnel, diplomats, university professors... What I've heard about the summer/winter courses is that there are students of all ages and backgrounds, however I can't tell from personal experience.

Thanks for the feedback. That is quite useful.

Hey. I have been to The Hague Academy twice: once during graduation for the Summer Session and this January for the Winter Session. They have a mixed crowd of students, varying among them with a different background. The ICL session has a targeted group of students, but that is not mandatory. I highly recommend the Hague Academy. Particularly I don't feel that the sessions are for 20s students to hang around because you go as an audit student so it is up to you to make the best of it. Feel free to ask anything more you want.