"That woman is much older than I am."
Translation:Esa mujer es más vieja que yo.
I used 'mucha más' and it was rejected. The English sentence uses 'much older'. If 'mucha más' is incorrect, then how would one convey the idea of 'much older' as opposed to 'older' in Spanish? To me, 'older' means anywhere from a few months to several years; 'Much older' means 10 or more years. I know that this is a subjective judgement, but I'd like to know how to make this distinction in Spanish.
In your example it would be "mucho más vieja" not "mucha más vieja". Mucho does not agree in gender or number when used as an adverb (as opposed to when used as an adjective). In this case "mucho" is qualifying "más vieja" and not "mujer" which makes it an adverb.
However be careful, when "más" is followed by a noun, then "mucho" can agree in number and gender of the noun.
Muchas más mujeres = Many more women. (In many dialects mucho más would also be acceptable here).
Las mujeres son mucho más viejas. = The women are much older.