Very interesting! Does it mean? The older person snipes at the younger person (opposite ***). The younger person is referred to as a diaper, because her/his fate is to change diapers. How about 'diaper changer' for the younger person?
It's a new one on me but I think you are misinterpreting it. The younger person is being joked about that they are so young they are still in diapers and a sniper is someone who targets another individual. So it means that the older person targets someone young. (similar to cradle snatcher).