"Dw i'n saith deg tri oed."
Translation:I'm seventy-three years old.
I'll spell out numbers if needed, but I think that "I'm 73 (seventy three)" should be accepted without "years old."
Comparing this to an earlier question where 'oed' was omitted, it seems that oed means years old (or at least years, or perhaps just 'old'). So, "I'm 73" might be valid usually, but in this particular case I think it was probably valid, I think, to mark it incorrect if you left off the 'years old'. Although in terms of normal usage you would be right.
Actually, I've found most of the other languages on Duolingo require numbers to be spelt out in full in English.
I've actually found the contrary. I always spell out the numbers in the target language and never get docked for writing the numbers in English.
Ah! Actually, maybe I just assumed it was also required in English. Worth knowing, thanks.