How to ask "is it really so that..."?
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？