You can say: "If I were two years younger than I am now, I would....

Also, simply saying "If I were two years younger" also works, as it insinuates that you are comparing it to your present age. "I'm 18. If I were two years younger, I wouldn't be allowed to smoke" If you want to be more specific, then you can compare your age to something else. "If I were two years younger than you..." "If I were two years younger than I am now..." You can also say "Two years ago, I would have...."