It's what we were told when DD was diagnosed (ADHD). My siblings were told the same in Australia and other parts of the UK.
Certainly most books I've read on ADHD will reference the 3 year thing. So not certain if there is a raft of peer-reviewed papers in the Lancet, but it's definitely widely known.
To be honest it's extremely helpful - even for just one's own sanity - when you are wondering why your child is incapable of lots of things.
And why I get really annoyed on Mumsnet and FB threads when you read things like 'students should be sorting their own choice of degree/completing UCAS form/task under consideration'. Some can, some shouldn't.
I'm having to do all the legwork for DD for universities - I can skim a webpage in seconds to see if it's useful, she'll be plodding through every word. So I am building her a spreadsheet with all the information she needs - module options, admissions requirements, size, open day dates etc etc so she can make decisions equipped with the facts. The universities have been more than happy to talk to me - I explain she is ND and will need information presented in a particular way and they are all fine with it. I am a complete dunce at her subject so won't be much of an influence!
Have friends with ND kids at Oxbridge and they have systems where parents of ND students know what assignments are coming up and when so they can nag from behind the scenes and help keep the show on the road.