Should not it be "sixteen years old" , plural , instead of year ,singular ????
In standard English, it's a set phrase: (number)-year-old (thing). Some dialects stray from this, but I believe Duolingo generally accepts only standard English.
The incubator tells me it's accepted, so I don't know what may have happened there.
The incubator says that's accepted too, but there may be a bug here. If anyone has a screenshot of it not accepting any of those, please share it with us so we can report it to staff.
I wrote "A sixteen years old boy" and it was accepted. Maybe it's only accepted "sixteen years old" and "sixteen-year old" ??
It tells me that the other combinations are also accepted, but the machine doesn't always work ideally.
That is correct, but it's extremely formal, bordering on archaic, but depending on where you are in the Anglophone world it may be preferred. "A sixteen-year old boy" is far more common.
Agree your solution is more modern, but the correct solution in American English is "a sixteen-year-old boy" (two hyphens) "https://www.quickanddirtytips.com/education/grammar/are-you-using-hyphens-correctly