I'd interpret this question as "Does this person think I'm experienced enough for the work?" over explicitly wanting to know whether I began at a young age.
When trying to quantify such a subjective characteristic we end up with a "Wittgenstein's ruler" situation where the question tells more about how the interviewer envisions someone programming from a young age rather than the interviewee past programming history.
Alternatively, are you asking how to determine someone has the claimed experience to match thier CV?