"Dw i'n saith deg tri oed."
Translation:I'm seventy-three 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.