I was thinking about it today, and there is another possible interpretation, same meaning but to a different audience. I am pretty sure Jesus was talking to the Pharisees when he told the parable, and I think that position is strongest grammatically, but there may be a chance He is telling the parable either for the disciples ears or for the Publicans and sinners or both groups. Basically telling His disciples " don't have this attitude the Pharisees are having towards sinners coming to me, and to the sinners, reassuring them. But the parable has the same meaning in this still. The prodigal is definitely the Publicans and sinners in the primary sense, but can refer to anyone who has lived a grossly immoral life and comes to Jesus, and the elder is primarily the Pharisees but can also refer to any religious person who judges and despises the little sheep who come to Jesus. The context is clear, and this interpretation is based on the setting, occasion, speaker, hearers, and clear intention of the parable
If anyone disagrees, feel free to give your view of who the prodigal represents and who the elder represents, and if you can provide contextual basis for your interpretatiin