In the UK I know there have been a few instances of people claiming to be minors, so they're less likely to be deported. I assume that 'age' would persist if they got leave to stay.
There are many such cases, not restricted to the UK. If you look you can find lots of rather amusing images about the topic. Photos of clearly adult men posing next to young teenagers, as if they are the same age. The amusing part is that everybody maintains the illusion.
But that is urban legend passed around illegal immigrants. Just watch "UK border force" or "Nothing to declare" on youtube. I am not from UK but it still was quite interesting thing to watch.