The comparative form of the adjective "old" is "older." Here are some examples of how to use the comparative form in a sentence:
Flora is old, but Frank is older.
The flora is old, diverse, and highly endemic.
These studies show that the species composition of the Tristan flora is old and has been stable for a considerable time.
Flora is old, but Tom is thinner.
The flora is old world and pantropic.
Flora is old, but the narrator is younger.
In each of these examples, the comparative form of "old" is used to compare two things or to describe something in relation to something else. The comparative form is used when comparing two things, while the superlative form is used when comparing three or more things.