I was referred to the cassel but never actually went. I went to the crisis recovery unit at the bethlem and it was a very similar programme to what they have at the cassel.
6 of us lived together in a large house (there was staff working there too, but only one or two nurses overnight) and we had group therapy throughout the day. Each day would start with a community meeting that everyone went to including all staff that were on. It would be a space to raise any issues that affected everyone and to be able to talk through out. Other groups we had were things like cooking (we'd plan a meal one week and budget for it, and the next we'd walk to sainsburys to buy the ingredients then walk back and cook it for everyone then eat), art therapy, creative writing, dance/movement therapy, weekend planning (we all went home at the weekends and so had to plan how to stay safe over the weekend and not engage in behaviours), swimming, DBT skills group. We also had individual therapy sessions. All meal times were regarded as groups too (and so no-one was supposed to leave without good reason and not eating or binging etc was regarded as self harm). Also every evening we had a group to sum up the day and discuss what had happened.
It was quite intense. The programme lasted 6 months. What I will say is that you need to be ready to make big changes if you go there. It will be incredibly tough at times. I was ambivalent about recovery when I went to the CRU and when I was there I started being in competition with the other residents and I wanted to be the most ill and self harm the worst. It hindered my recovery and I acted out and got sent home a few times. When my time there ended I was sad that I'd wasted so much of it. I made progress, but not as much as I could have done. Although as a result of being there I never self harmed badly enough to need A&E treatment again, and within 1 or 2 years I stopped self harming completely barring the odd slip up. Over the years thanks to that admission and another one where I did more DBT I recovered enough to not fit the criteria for BPD anymore. I went from not being able to study or work, needing to be inpatient because my self harming was bad and I was frequently suicidal to living independently, studying full time and working part time, discharged from the CMHT and not self harming any more.
Don't be concerned about your age. At the CRU I was by far the youngest as I was 18 when I was there. A few others were in their 20s-30s and some were 40-50. My advice is to keep talking- don't bottle anything up as it turns to resentment or other negative emotions. In such an environment it's inevitable you wont get on with someone or there will be an argument, and it's best to deal with it as it happens and sort it out rather than have to live in an unpleasant environment. It might be a bit of a culture shock at first living with other unwell people but you will settle down and start to cope with it.