My husband boarded from 7-13, as did his brother (2 years younger). They were choristers. The musical training they got is unsurpassed and they have great memories. Their parents found it very hard to be away for them for so long, as they were living in another part of the country and one of the parents is clergy so works every weekend all day Sunday. They also still have strong close friendships from that time.
There’s now a movement away from choir schools that require boarding all week and most of the weekend. Magdalen College choristers, some of the best in the country, no longer sing Saturdays - women replace trebles for that service - so they can spend time with parents. Canterbury Cathedral has just announced they are no longer only taking boarders and will instead take day pupils from schools around the area. So it’s definitely falling out of fashion and there is probably a recognition that boarding and very intense musical training is not good for a lot of young people.
Interestingly my FIL is very critical of himself for sending DH and BIL, but BIL and DH really loved their experience and are proud of it.
I went to a wedding this weekend and the groom - who boarded from I think 13 to 18 - paid the most wonderful tribute to his parents. He’s incredibly close to them and has been throughout his childhood and school years. There was no tinge of regret at the ‘cruelty’ of boarding school - just love for the amazing opportunities they’d given him.
That experience is common although not universal among friends who boarded (quite a lot of my friends did). They also mostly found it easier to go to university - less homesickness, more independence. Not a universal experience of course. Of my friends who boarded, more of the girls seemed to find it difficult than the boys.
So no, not everyone is damaged and it isn’t a terribly cruel experience for everyone. I don’t think I would choose boarding at age 7 for my kids, but I don’t think it’s wrong for everyone. I do think they should be able to frequently see family. The foreign students left in the boarding houses on exeat (sp?) weekends seems very sad.