Absolutely.
An ebook does not stop being promotable because it has been available for months or years. A promotion can introduce an older title to readers who have never encountered it before.
Older books can be especially good candidates when they are the beginning of a completed or established series, have accumulated strong reviews, or have recently received a new cover, description, or pricing strategy.
For many authors, the backlist is one of their most valuable marketing assets. A well-timed promotion can give an older book a new opportunity to reach readers.