Lucas

Maybe and this is of course a reach. But maybe owen left the farm for a little bit. Found Beru and fell in love. Only to return to the farm when Shmi was kidnapped. So technically he might have only been on the farm a month or so working to help our his now disabled father.

Or

He had a silver leg so Owen wouldn't remember him then. If you recall in Ep 7 when C3p0 sees Han he was like you might not recognize or remember me because of my red arm. lol Owen was like we never had a droid with a silver leg.