“UN ANCIANO” IS NOT "AN ANCIENT"
I have experienced this mistake in a few of my classes. To be honest, I have not heard many people use the word "anciano" in daily life here in Spain, but clearly, it is a sufficiently important word to have an impact on some of my student's English!
So, you can not use the word "ancient" in English to describe an old person!! In fact, even the word "old" is not ideal because "old" people could potentially be offended by it! A better option is: "elderly". The word "ancient" can be used in English but is more commonly connected to "ancient civilisations" or "Ancient Greece"
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 10 años de experiencia enseñando inglés a españoles.