If I were 5 year older?
You told me was is singular and were is plural, but if I were sounds funny.
A. Was" is singular and "were" is plural in ordinary statements and questions, but in many "if" clauses, we use a form of verb called the subjunctive. I wish I were with you. This sentence is right because it has to do with a wish. The link below explains a little more about the subjunctive.