You're asking for a number. Your interviewer either gives it to you, or he evades.
Another brilliant one is "how long are you planning to stay?".
This can lead to some of the most revealing and unstructured discussions you'll engage in during your interview process. Not for the feint of heart.
For example, if you join a small company to lead, say, their first Android application's development, and one of your first tasks involves hiring another developer or two to round out your team, there's not going to be a lot of tenure there. That certainly represents a risk, but it doesn't indicate something wrong with the company.
When I encounter situations like this, I often ask about the team members' prior experience. They're not obliged to share those details, of course, but most people are eager to tell you about their past achievements and how they hope to outdo themselves in their new roles.
The second and third questions are good, but short tenures could also signify a rapidly growing company (which is something to be wary of, but not for the same reason) like Google in 2006.
Sure, but if everyone interviewing you are less than a year in, where are all the people who worked here before?
And why are you being interviewed mostly or exclusively by junior team members who haven't proved themselves yet? Is the company entrusting the selection of successful employees to newbies who haven't yet proved they can be such?
An interviewer for a company in the middle of a growth spurt should be able to explain that, quote some enticing growth figures, and mention that "almost all of the original team members are now project leaders and executives... in fact, if this goes well, you'll meet some of them shortly".