Most phobias start in childhood, but they can also develop in adults.
Explanation:
In the given sentence, a comma between the noun <em>childhood </em>and conjunction <em>but</em> is necessary.
This is a compound sentence. A compound sentence is a sentence that consists of at least two independent clauses joined by a comma, semicolon, or conjunction. When we use a conjunction to connect two independent clauses, it should be preceded by a comma. That's why the original sentence is incorrect.
