Grammar question

[Eric Laursen]

Vsem deviatnadtsat' let.
Vsem po deviatnadtsat' let.
(Everyone is 19 years old)

Can anyone tell me the difference (is one more colloquial, etc.).  Also,
what case is "po" taking here?   Is this dative, and the number is just not
declining?  (One native speaker said it would be "Vsem po odnoi nedele").
Or is it accusative.   Thanks in advance.

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