"He has been taking care of a child since he was eighteen."
Translation:Od osmnácti let se stará o dítě.
I was wrong on this one, with "Stará se o dítě od svého osmnáctého roku." The translation in my head was "He has been taking care of a child since his eighteenth YEAR," so I used genitive singular. The correct answer offered, however, appears to be genitive plural -- Stará se o dítě od svých osmnácti roků -- and I don't know why. Could someone explain, please?
Meanwhile, since I think I've gotten this one wrong before, I should probably get the answer above (Od osmnácti let se stará o dítě) locked into my head...
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UPDATE -- Thanks to both of you for your replies, and for the addition. Eventually, I'll master the right way to talk about age...
To keep it short: "Stará se o dítě od svého osmnáctého roku" is grammatically correct and possible, however, it's not a common way how to speak about age.