Translation:I am fourteen years old.
This can also mean "It is fourteen years old," although that would sound strange in any context. Primarily, though, when no subject is directly named, it can be safe to assume that the speaker is referring to themselves. Again, due to the subjective nature of language, pay close attention to the context of the situation.
Also correct me if this is wrong, because I am still new to 日本語。
(じゅうしさい is also technically correct, but you just won't hear that as often, partially because of popular convention and partially because of superstitions surrounding the し reading of 四.)
The character for "death" (死) is also read as し. As such, the number 4, in particular the し reading of it, is considered unlucky in Japan.
The same is true, to a lesser extent, with the く reading of 九 (9). It's read the same way as 苦, which means "pain" or "suffering," so people tend to favor the きゅう reading instead.
I knew about the し = death thing, but not about the く = pain. Thanks for the info!