• 1 + singular Nominative (один рік)
• 2,3,4 + plural Nominative (два, три, чотири роки)
• >5 + plural Genetive (п’ять років)

Somehow >5 is so big it becomes "five of years" x) Sorry, I don't have any excuses or explanations, just a weird pattern we follow when we think/talk/write...