What is the function of "o" in this sentence: "Před rokem jsi byla o rok mladší."?
It is used when comparing things by a value. "o centimetr vyšší", "o něco hubenější", "o den víc".
I would loosely translate is as 'BY'. You are older BY one year. Sounds weird in English but might give you a general idea
Which declination does it take?
o takes the accusative case here
Why can't you say "Last year, you were a year younger."?