Obviously. Rollo always gave off serious younger brother vibes since S1. A guy his size, if he was the older brother he wouldn't cower like a child when Ragnar berates him, that kind of dynamic stems from childhood.

And the way he tries to assert his authority in the pilot by getting in Ragnar's face and calling him "little brother", while Ragnar simply enforces his command again like he's dealing with a pouting child, then throws a tantrum by breaking everything for not getting his way, typical younger brother move.

Also Ragnar inherited the familial farm, that always goes to the oldest son.

Idk it always seemed evident to me he was younger.