If one direction were all 16 when they auditioned, then how are some older then others?

Three members are 18, while one is 20, and the other 21.
they all claimed to be 16 when they auditioned for Britain's got talent.
They all auditioned in the same year...
Yet some seem to have aged 3 or 4 years since then, while others have aged only 2 years.
It's been only 2 years since they were first formed on the show.
How does this work??

uh, publicity!

i think louis was 17-18 when they auditioned? idk