"Estimated to be" versus "estimated at..."

[jhuman lal meena]

that's a lot of words.

here's what is actually going on here:

* "at" is a preposition.
* prepositions must be followed by nouns.
* "xxx number of years old" is not a noun.
so, wrong.

in the other example -- "temperatures estimated at xxxx degrees fahrenheit" -- there's no problem, because "xxxx degrees fahrenheit" is a noun.

i don't think anything else is happening here.
the OG explanations are wrong fairly regularly (though not in most cases); this is one of those times.

Good try Ron, but don't you think that "old" can be a noun too.
Sentence: Old is gold.
Here old is the subject of the sentence and thus a noun.

[tim]

Not on the GMAT. Just because you make up a sentence that sounds good to you doesn't mean it's correct. Your example is grammatically incorrect *precisely* because "old" isn't a noun. Putting a word in a sentence where a noun goes can create a memorable phrase, but sometimes the most memorable phrases are memorable because they are purposely made to be grammatically incorrect.