As and when. In their 'original intended career' you're going to be better off as an acturay than an accountant.
On the other hand, most accountants don't stay in accountancy for longer than 3-5 years. They go somewhere that 'pays more and works less'. I'm not sure how 'career mobile' actuaries are, but accountancy is a profession known for it. Whilst there are more accountants in high places than actuaries in high places, that's partially to do with the fact accountantas are ten a penny. BUT, i'd imagine the transferable skills and business exposure accountants pick up is amongst the highest of any finance professionals, as they get to work with so many clients 'on site'.
Also, most accountants regard their 3 years of exams as a royal pain in the arse. Actuaries would, i assume, kill to have only 3 years of exams.
By looking that which pays more, you're taking a very short term view. Accountancy partners are on £500k+, I'm sure roughly a similar proportion are on similar money. An actuary might be on £60k after 4/5 years. So will an auditor if they go into industry. Base your career on something a bit deeper.