I was slapped on my left ear as a small child. It didn't cause a hearing problem, to my knowledge. Now in late life I have age related hearing loss and the left is far worse than the right. Can this be as a result of the slap years ago?

Yes it could, but it is likely it can not be proved now. There may be many reasons. Now not time to worry what happened years ago, but to get treated an try to heal hearing