Justin Greer has already given an excellent answer, but it’s worth looking at why some examples of this seem more marked/forced, while others (like W2’s comment on the question) seem rather more plausible.
The most obvious way to get than at the start of a full declarative sentence is to use a “PP-fronting” construction, i.e. putting the prepositional phrase “than …” at the start, where you would normally expect to find the subject of the sentence.
So the key is to notice when and why English uses PP-fronting. It gets used mainly for topicalisation: that is, taking a phrase which would not normally the main topic of the sentence, and making it the topic. (See: the topic–comment model; and a couple of papers on PP-fronting.)
A sentence that fronts a “than…” phrase, then, is going to sound more natural if there is a clear reason for the phrase to get topicalised. One very strong natural reason is if it’s being contrasted with a parallel phrase in another sentence, where the rest of the sentence stays the same.
Beethoven is perhaps a greater composer than Mozart. Than Bach, though, he is certainly not greater!
Another way to get than to the front is to have the subject of the sentence a noun phrase which, by ellipsis, begins with than:
Running faster than a cat is easy. Than a dog, though, is more difficult.
Here the subject is the noun phrase “[running faster] than a dog”. So this example does rely crucially on its context, with the previous sentence supplying the ellipsis. It’s still a fully grammatical simple declarative sentence, though!