Till is indeed older and the two can be used pretty much interchangeably.
There is really only one usage I can think of where you would not use till: in setting up a negative conditional.
Until my landlord fixes the plumbing, I am not going to pay the rent.
I am not going to pay the rent until my landlord fixes the plumbing.
Here it would sound strange to use till, and I’m not really sure why. Perhaps it is the negative.
One more thing. Sometimes you will see people spell till as a contraction of until: that is, as ’til. This is simply wrong, and not seen outside of old poetry and modern greeting cards aiming for a “poetic” tone. Anywhere you see ’til being used, till would be the correct word.
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