Brainteaser Approach?

Pretty straightforward: make an assumption about the number of ordinary years in one light year and multiply the age difference by that number. You can try and take it one step further by factoring in some adjustments for measurement error or scientifics but you'll have to research it on wikipedia.

? lightyear is a measure of distance not of time or anything like that. Light travels at approximately 300,000,000 meters per second. There are 606024*365 seconds in a year, which is 59,130,000, approximately 60,000,000. 1 light year is therefore 1.8 *10^16 meters and assuming you don't smell you are about a meter from the interviewer.

If you are given this during an interview estimate 50 million seconds and 300 million meters per second and go from there.

I would round down to zero. Next question.

Lightyear is a unit of measure... look down at the space between your and the interviewers feet and say "Since a lightyear is a unit of measure I would say about 4 feet?"

a light year is how far light travels in one year. light travels at 310^8 m/s so in one year it will travel that times 360024365 that. since when your interviewing you are probably 4 or 5 meters apart, you are 5/(310^8 * 3600* 24 * 365). If you reason it out that way I bet they'd be content...then just say that numbers is something like 1 in 10^13

4 or 5 meters apart? So your interviewer is 15 feet away from you? Yeah that makes sense