Learn Spanish (Latin America): Yo soy mayor que mi hermana menor.

While viejo means "old," Spanish uses the irregular word mayor to say "older" when talking about people's ages. Saying más viejo isn't grammatically wrong, but it can sound a bit rude (like saying someone is "more elderly" or worn out), so mayor is the standard way to say "older."

Why is it mayor que and not mayor de?

When you are comparing two things directly using "than" in Spanish, you almost always use que. So, mayor que translates exactly to "older than." You usually only see de used with numbers or in phrases like "the oldest of the family."

Wait, doesn't menor mean "younger than"? Why doesn't it have que after it here?

Words like mayor and menor can be used in two ways. They can act as comparatives comparing two things (like mayor que). But they can also be used as regular adjectives meaning "older" or "younger." In hermana menor, menor is just acting as an adjective describing the sister, so it doesn't need que.

Is it ever correct to say más mayor or más menor?