[cruise02]

"...to a 5 year old" should be pretty safe. "...to your grandmother" could be construed as both sexist and ageist, so I'd avoid it.

[CaptSpify]

Personally, I don't really see much of a difference. Some 5 year olds are better at $x than "grandmothers". So wouldn't "to a 5 year old" be ageist as well?

[cruise02]

Use some common sense. For how many values of $x that would be the subject of an interview question are 5 year olds going to be better than grandmothers?