My answer's the same as Bera's and Phils. It usually means 'You're getting old". I've never hear it worded this way where I am, though. It's usually said as a statement, not a question. ie "You're getting on a bit". as if to say, "You're getting on in years"/"You're getting old". Sometimes it's said as a mild joke to say you're *looking* old these days. You do have to be careful how you say it, as it could be misinterpreted as a criticism/mild insult as if to say that time is not treating you kindly.

Can’t say for sure without the context, but my guess is the same as Bera’s, that is, that it’s short for “getting on in years a bit”. You can find this (American?) idiom in the “Free Dictionary”: https://idioms.thefreedictionary.com/get+on In British English, there are a few additional meanings, but again, you need the context to figure out which one it is. Here’s the entry in the Cambridge dictionary, which includes several possible meanings (and indicates UK or American usage): https://dictionary.cambridge.org/dictionary/english/get-on Remember: If you want a good answer, always provide the context.

Can you stay a little bit.

Are you getting old