通过减去R中的年数来计算每只动物的年龄

I am looking to calculate relative age of animals. I need to subtract sequentially each year from the next for each animal in my dataset. Because an animal can have multiple reproductive events in a year, I need the age for the remaining events in that year (i.e. all events after the first) to be the same as the initial calculation. I am looking to calculate relative age of anima