The Chief Inspector of Prisons has said he "fears for all young offenders" after a visit to an institution in West Yorkshire.
Reported on Wetherby Young Offenders Institution , Nick Hardwick described how one young boy tearfully asked to be taken home to his mother while another, described as "low", lay on his bed not speaking.
Some of the most challenging of the 340 boys held at the time of the inspection, most of whom were aged 16 and 17, were also the most vulnerable, he added.
"Walking round the establishment, the vulnerability of some of the young people held was obvious," he said.
"One boy in the segregation unit with a lifelong medical condition that would have been hard for any teenager to manage, and who had exhibited very disruptive behaviour, asked me tearfully if I could take him home to his mum.
"I was later told he had been moved to a more appropriate secure medical facility.
"Another boy, who looked about 12 and was sporting a dramatic black eye, had been convicted of a serious offence, had been in further trouble and was confined to his cell.
"A boy in health care, described to me as 'low', lay on his bed not speaking. All these boys were receiving good attention and care, but you feared for them all."
A few days before the inspection at the end of January, two boys had died elsewhere in custody.
And in April last year, Ryan Clark, 17 was found hanging in his cell at Wetherby, where he was being held on remand.
While Wetherby provided "reasonably good outcomes" for most young people overall, he said there were weaknesses which needed to be tackled by both the institution and the Government.
Some young people received no or very few visits, with about half of them being held more than 50 miles from home and one in seven (14%) more than 100 miles away.
A unit was opened at the institute in 2008, and was designed to offer a safe environment for young men who could not cope in the mainstream prison system.
But Penelope Gibbs, director of the Prison Reform Trust 's Out of Trouble programme, said: "It cannot meet the needs of all the vulnerable boys in this one prison, let alone all those imprisoned in England and Wales.
"This poses a challenge to the whole system, and to the appropriateness of imprisoning any under-18 year olds in a prison service establishment."