Leah G. answered 10/25/22
This is a great question and highlights some challenging ideas in the English language.
Writing a story in the past.
Your statement "Winter in Toronto is always cold" is a great way to invite your reader into the setting of your story. In fact, it IS always cold in winter. That does not change. So you can talk about it this way, i.e. using present tense. It also indicates that you are likely presenting a narrative. Narratives are always told in the past because they have already happened. So switching to past tense at this point would still work. For example,
Winter in Toronto is always cold. That is why Susan would never visit her sister during December.
It's a scene setting sentence.
The second question regarding how to talk about age needs a closer look. The way you have it, ""Lisa was raised in Califonia, now she was 20 years old"? Or I should say: "She was 20 years old at the moment"?
would be incorrect.
The sentence should be rewritten for clarity. For example, Lisa was raised in California and currently is 20 years old.
She is either 20 years old or she is not. If she is older than that NOW, then you must say "She was twenty years old in 2021 (or whatever). Example: Lisa was raised in California and was 20 years old when she discovered she had a sister in Ohio. Here, the context makes it work.