In Hebrew numbers are written out in their component parts; there's no Hebrew word for "990", for example, but the torah writes instead "90 and 900" (or sometimes "900" and "90") as separate words. Everywhere in the torah that you see numbers in the hundreds or thousands, you'll see this formation.
As for "ben", this is a Hebrew convention when talking about people's ages. In English we say that somebody is "X years old"; in Hebrew the "old" isn't there but "ben" (literally "son of") is used instead. It would be incorrect to say just that somebody "is 990 years"; English and Hebrew use different conventions for addressing that. This Hebrew formation is not unique to this passage, as Scimonster pointed out in a comment -- see also 11:10, 25:20, 32:2.
Sometimes ages are given without the "ben", but in those cases there is another verb. For example, the genealogies in chapter 5 all say "and so-and-so lived X years".