How to ask "is it really so that..."? How do I ask a question like "is it really so that you're just 18?" or "is it really so that the movie was good?", i.e. a surprised question?

My guess is something along the lines of "这是真的，你只有十八岁呢?", but it sounds weird.
It sounds weird because there is no formal subject (A.K.A. anticipatory subject) in Chinese. It is [X] that [Y] should be transformed into [Y][verb as appropriate][X] during translation。For example:

It is a pleasure to meet you. =&gt;
Meeting you is a pleasure. =&gt;
见到你很高兴。


So for the original sentence,

Is it really so that you're just 18? =&gt;
Are you really just 18? =&gt;
你真的只有18岁？