This is something that confuses a lot of native speakers. It seems Romanians use both. This article tries to make sense of it, basically claiming that the plural is used when the emphasis is on the individuals and the singular is used when the emphasis is on the group. Indeed, this seems to cover how speakers use singular/plural intuitively (based on my own observations).
These age translations don't follow proper translation practices. You are literally saying "The majority have seventy years", not "The majority is seventy years old". There is a difference between meaning and translation. It should be translated correctly, followed by what the translation means -- not translated incorrectly to teach meaning.
There may be some regional dialect of American English in which this is natural, but not in any of the regions in which I have lived very long (the Pacific Northwest, Southern New England, Eastern New England, New York City, the Chesapeake Bay, the Ohio Valley, and the Southern Great Lakes).
And I've lived in the Seattle area, multiple parts of the Midwest, and in Florida. It's not common, but I've heard new parents describe their baby's age this way numerous times. Regardless of how "natural" this sounds or not is irrelevant. "She has 3 months" is how they describe age, so that's how it should be translated. You're not breaking the English translation by doing so -- you're introducing a new way of describing age. Again, translation should come before meaning, not completely replaced by it.
"translation should come before meaning" is a meaningless sentence. If the result of your "translation" doesn't have a meaning, it's not a translation. At best, it's called glossing, and it can be useful to teach sentence structure, or as a first step before translation proper.