Calculating age per animal by subtracting years in R - r

Question

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.
Update:
The dataset more resembles this:
Year ID Age
1 1975 6 -1
2 1975 6 -1
3 1976 6 -1
4 1977 6 -1
6 1975 9 -1
8 1978 9 -1
And I need it to look like this
Year ID Age
1 1975 6 0
2 1975 6 0
3 1976 6 1
4 1977 6 2
6 1975 9 0
8 1978 9 3
Apologies for the initial confusion, if I wasn't clear on what I needed to accomplish.
Any help would be greatly appreciated.

Answer 1

Things done "by group" are usually easiest to do using dplyr or data.table
library(dplyr)
your_data %>%
group_by(ID) %>% # group by ID
mutate(Age = Year - min(Year)) # add new column
or
library(data.table)
setDT(your_data) # convert to data table
# add new column by group
your_data[, Age := Year - min(Year), by = ID]
In base R, ave is probably easiest for adding a groupwise columns to existing data:
your_data$Age = with(your_data, ave(Year, ID, function(x) x - min(x)))
but the syntax isn't as nice as the options above.
You can test on this data:
your_data = read.table(text = " Year ID Age
1 1975 6 -1
2 1975 6 -1
3 1976 6 -1
4 1977 6 -1
6 1975 9 -1
8 1978 9 -1 ", header = T)

Answer 2

if you're trying to figure out the relative age based on one intial birth year, 1975 (which it seems like you are), then you can just make a new column called "RelativeAge" and set it equal to the year - 1975
data$RelativeAge = (Year-1975)
then just get rid of the original "Age" column, or rename as necessary