(Bob avoids an anuerism. A near miss.

)
If they are serious about saying their age is way different than what it obviously is -- then they're nuts, yes. I would keep an eye on them.
If they are just a little nutty, I would just ignore it. I've found with students and some people I have to very much "turn my brain off." Esp if I'm not around them for some time. Because sometimes they say things that make no sense to anyone but them (if that), but my mind likes to make sense of things. I had to concentrate at first to do this -- to not think much about the strange thing they just said. Just add a comment like "Wow, that's interesting," or a "hmm" and you avoid agreeing to something. It is definitely a teaching and people skill.
If she's 50 and says 29 I would give her a puzzled look at say "You're 29?" Then laugh and say "I thought you were 37 before?" Nicely question.
Fifty though -- It could be the mind going. Or it could be something she does and thinks is cute, or helps her cope. Who knows?
This thread seems kind of fishy to me. I don't what to think about it.
If she keeps this up she'll be a teen again, then into the single digits, and then pretty soon you won't have a student at all.