From The Daily Mail:
Why it pays to do maths A level: Analysis shows qualification adds £6,000 to a salary in just six years compared to geography or biology
So, if a child is considering whether to do A-Levels, and if so which subject, they should choose Maths?
Nope.
To a large extent, this is confusing cause and effect and ignoring self-selection.
Some people are more numerate than others; and those who are are, are more likely to earn more . Either because their job required advanced numeracy (very few jobs, if truth be told); because numerate people are more likely to be more efficient and hence be promoted; or because they are clued up enough to choose a job/career that will pay more in the long run (so they waste a few years studying or doing a low-paid apprenticeship in exchange for higher pay later on).
People who are numerate are also more likely to do a Maths A Level, obviously, even though the maths involved is insanely arcane and probably only of use in 0.1% of jobs.
So... numerate people are more likely to end up in higher paying jobs. They will be over-represented among applicants (which they would have been anyway); will be slightly better at them (as they would have been anyway); and employers will tend to prefer applicants who have done 'hard' A-Levels (however pointless, and as much as I love numbers and maths, even GCSE is way more than most people ever really need in real life).
Or to turn the question round, a child is not so numerate. Should they choose Maths A-Level and almost certainly fail? Hell no. Better to pass in something else a bit softer.