Here is a website that will calculate how old you are -- not only in years, but also in months, weeks, days, hours, and seconds! It also counts down (by seconds) to your next birthday. Warning - this may be depressing to adults!
You might want your students to estimate how many minutes (or hours or seconds) they are old, or maybe even have them try to calculate it themselves. Then they can go to the website and see how close they were!