Yeah I wracked my brain for a few years, every now and then when I was in school. I wanted to find an equation to figure out when I would start getting closer to my Dad's age. I finally figured it out, although I don't know how to do the equation bit. Essentially, you become HALF as old as your Dad once you turn the same age as they were when you were born. So if he was 20, then when YOU turn 20, he'd be 40. From that moment on, you'll be that number of years apart, but the age gap compared to 0 decreases by 1 year each year. However, since they age 1 year each year too, you'll never catch up until they pass.
Something else that was cool with the year I was born... 1980. So in 1981, take the last digit and that's how old I am: 1. 1985, 5. Then comes 1990. It's a 0, but that just means I'm 10. Then comes 2000. Easy, take the 1st digit and last digit.. so 2000=20. 2008, 28. Once we hit 2010, it was add the first 2 digits with the last 2 digits.. so 2010, 30. 2017, same thing.. 37. I got it easy, I'll never have a hard time figuring out how old I am. 2080 would be 100 years. 2090 (like 1990) would be 110. Just add a 1 to the first number and start again. So if I live to 2100, it should be 120. That's where it gets confusing. because 2105 would only be 125.