Why do we say "30 years old" but "a 30-year-old man"?

This pattern goes all the way back to Old English (alias Anglo-Saxon).

It's the same reason many of us say that someone is "5 foot 2" rather than "5 feet 2". The source of the idiom is the old genitive plural, which did not end in -s, and did not contain a high front vowel to trigger umlaut ("foot" -v- "feet").

When the ending was lost because of regular phonetic developments, the pattern remained the same, and it now seemed that the singular rather than the plural was in use.

from alt.usage.english / Mini-FAQ on Grammar, Usage and Punctuation