The two brothers -- Matthew, 39, and Michael Clark, 42 -- contracted terminal leukodystrophy last year. It usually only affects newborns and despite having jobs both now behave much like toddlers, according to news reports.
Michael, an ex-Royal Air Force regiment member, is now thought to have a mental age of 10 and giggles constantly while his elder brother Matthew, a factory worker, behaves like a small child.
Their parents Anthony, 63, and Christine Clark, 61, are devastated. Anthony, from Lincoln, Lincolnshire county, said: "It is a devastating disease. Both of them are very childlike now. Matthew went out the other day and bought himself a train set and a Mr. Potato Head."
"It is like an adult having a toddler's tantrum. It's obviously worse for him but it is terrible for us too. We feel absolutely powerless," he said.
Michael's symptoms turned more apparent as he was evicted for not looking after himself.
"Matthew was fired from his job as a factory worker, and although we don't know the full story we can only assume it is because he was behaving like a child," said the father.
The parents have now abandoned their plans to retire to Spain as they have to look after Michael and Matthew, the grown ups turning kids.
"There is an estimated 1 in 3 billion chance of two people who carry the gene deficiency meeting and becoming partners," said Lynda Carthy, British chief executive of the Myelin Project which funds research into the disease.
Leukodystrophy is a neurological disease which affects the brain, nervous system and the spinal cord. Little is known about the condition.