Going out for a meal to celebrate your birthday is a fine tradition, and it looks like whoever left this note had a great time on their special day. It also appears that whoever served them helped to ensure they enjoyed themselves, and so they got their reward!
We're not sure about their system of basing their tip on their age, though. That gets more and more expensive as you get older. We bet that the restaurant is hoping the customer lives to be 100, and then comes to celebrate the occasion with them.