DH boarded from 8.
I boarded from 11.
DH, oddly, loved boarding as a young child - he saw his parents only for the long holiday, as he was sent 'home' from abroad to board - but didn't much like 13+ as a group of less-congenial boys arrived who he didn't get on with. I would not say he is 'damaged' by boarding. His relatioonship with his parents is / was not close, but the reason for this is much more that, when his parents returned from abroad to live in the UK, his sister was removed from her boarding school, lived at home and was found a day school, while he was left in his boarding school. The feeling that 'they want her company but not mine' - an entirely suitable day school for him was available, but ever offered to him - means that, to thios day, SiL's relationship with her parents is much closer and warmer than DH's is.
I boarded as it seemed at the time to be the only way to get a suitable education. I was a 100% scholarship / accelerated by a year / Oxbridge PhD type. The local rural secondary had just stopped being a secondary modern and its ability to teach highly able children was, at that point, unclear [my brothers went on from there to Oxbridge, but that wasn't forseeable at my point of entry IYSWIM]. No academic day school was within commuting distance.
It was always very clear to me why I was there - I knew and relied absolutely on my parents' love and family bonds, but knew that my 'task' was to be well-educated. yes, that sounds a bit bleak. it was, on occasion, a buit bleak. But it wasn't emoptionally damaging, and i did, educationally, get out exactly what Ineeded, for which I am very grateful.
Our children are state educated non-boarders - which perhaps tells you more about our 'gut feel' about boarding than any of the above!