If you can say "she is xxx years OLD" for "ella tiene xxxx años", why is it wrong to answer "she is more than fifty years OLD" for "ella tiene más de cincuenta años"....
I'll grant you that "over" usually sounds better, but traditionally "more than" is more correct grammatically. My journalism professors in grad school were adamant about it. I think it's relaxing these days, and it's constantly used that way: http://grammar.quickanddirtytips.com/more-than-versus-over.aspx
It would seem DL is making a distinction for like brevity of phrasing by linking xxx años to xxx (in english) and xxx años de edad to xxx years old/of age. I have seen both accepted in other like sentences where apparently reported, but does not seem to be part of the algorithm rather entered sentence by sentence.
Yes it's just something that exists in Spanish that we can't translate literally in this case. To refer to age in Spanish we use "tener + age", e.g. Tengo veinte años=(literally "I have twenty years") I am twenty years old. http://spanish.about.com/od/idiomsandphrases/a/age.htm
It's because you can't do that in this case. You have to translate "tener + (age)" idiomatically in this case, you cannot translate it literally. http://www.spanishdict.com/answers/107664/how-do-you-say-the-age-of-someone