"Those streets are old."
Translation:Ty ulice jsou staré.
In some previous examples, "To" was used instead of "Ty". Does it matter that much?
In this sentence ty is a demonstrative modifier of ulice, indicating that we're talking about particular streets -- "those" ones. You might also see a sentence like "To jsou staré ulice." That would mean "They/those are old streets," while what we have here is "Those streets are old."
I would say it depends on the order of words in the sentence. One says "to jsou staré ulice", or "ty ulice jsou staré". But, one can't say "to ulice jsou staré" because "ulice" is feminine. (I couldn't say, however, if "ty jsou staré ulice" is wrong or not.)