B.K.
It's at least 12 months old AND at least 20 lbs. Not whichever comes first but once they meet both minimums. However, it is safer to keep them facing backwards as long as possible. If I am not mistaken, it has something to do with strength in their neck. The longer you keep them backwards, the better. Mine stayed backwards until they were 14 or 15 months and 23 lbs or so. I could have gone longer but chose not to.