I am a 23 year old User Experience Architect and I've been working in the IT field since I was 16 years old. Before accepting my current employment, I encountered a firm that was big on age/experience and was comprised of mostly engineers in the 35-40 age bracket who had been working in the industry for 10+ years. I was asked a question almost exactly like the one you postulated.
Using simple math I constructed the following response, which I believe caused the interviewer trouble with the big wigs in the room:
I've been working in IT for about 8 years now of my 23, where as this older gentleman has been working for 15 of his 45. Since we have both been in the industry gathering career experience for the exact same percentage of our life experience, the question should be about quality and adaptability. Not quantity.
I wonder what happened to that guy...