Don't think the old gas in the mower nor the gas in the can had anything to do with the splash-back.
Whatever you did to cause it needs to be addressed.
More details would be needed to help you avoid the same happening again.
Avoid pouring so fast, or avoid having face in path of the splash.
Your eye rinsing was a good plan and hope you can find a better way next time.. Albeit, fresh gas is the way to start the season.