older

Older means greater in age than somebody or something else. My older brother, an older building, three years older.

It is the ordinary comparative of old, and it is the one to use for people: my older sister. Elder is the alternative and is restricted — it works before a noun within a family, as in my elder sister, and cannot be used with than. So older than is correct and elder than is not. Note the older, meaning older people generally, and that older person is now the preferred polite term over elderly.