Alternative and related questions:
Why aren't you earning more at your age?
Do you think you are being paid enough?
The meaning behind the question:
This is a two-pronged attack. The interviewer is putting you under stress with what is obviously a rather awkward and aggressive question and, secondly, they're looking to see how you justify yourself and the career path you have taken.
Your answer:
As with other 'tough' questions, it's important to remain calm and not to become defensive.
The best approach is to justify the career decisions you have taken because they have led you to where you are today and put you in a position where, indeed, you do now expect to earn more than you have done previously.
Turn the question round on them. Go from a question which implies a weakness to an answer which demonstrates your strengths.
Example:
Rather than focus on short-term earnings, I have been more focussed during my career to date on gaining a broad variety of marketable skills and experience. I've been focussed on developing within my line of work and have consequently deliberately chosen certain jobs, not because of the financial package but because of what I would learn from them. I've certainly never changed jobs just because of financial incentives. Training and development opportunities have, to date, always been more important to me. However, I'm not saying that I'm not now very keen to realise my worth. I fully recognise my current market value and achieving a respectable market value has been my long-term goal. This is why I have taken the career path I have.
Word of warning:
Without being overly defensive, you should definitely justify your career decisions. Don't let the interviewer trick you into admitting you've made mistakes along the way.