"Dw i'n naw deg un oed."
Translation:I am ninety-one years old.
In this context, the ... oed on its own is sufficient to mean '... years old'.
See the notes - https://www.duolingo.com/skill/cy/Numbers2/tips-and-notes
Me too: ibisc says that the ...oed is sufficient to mean ...years old which , to me, means that it goes without saying. The English version, 'I'm 91' does the same thing. The 'years old' bit is silent but understood. Surely it should be marked as correct?