It's an alternative form of 'årtiene'.
Ok, nice. Tiårene = årtiene. Might one of these be 'old-skool', is preference regional or are both used widely?
tiårene is roughly 30x more common than årtiene and 1000x more common than årtia. There might be a few dialects which prefers the uncommon ones, but I'd just avoid them.
årtiene might also be confused with årstidene when one is a beginner so I agree with tiårene/tiåra being the best choice for beginners.