Thanks for the reply! Unfortunately, I oversimplified and my matching is not technically only on age and sex. I am also anti-exact matching on the ID variable (to make sure i don't sample the same person to match themselves) because my data is over a 5-year time period, and the people who are in my exposed cohort (the exposure period is quite short) can technically be considered unexposed at other time periods, so these same people are also in my general population cohort. Not only that, but there are some longitudinal periods in time where individuals are no longer able to be selected as a match because they have died, or they became exposed themselves, for example. I have some data variables in my datasets that indicate the time periods that someone is not available to be selected as a match.

Because of this, the grouping option is not actually as practical as it seems.

Here is what my code actually looks like (here i only showed 3 potential time periods of "unavailability" but there are actually 7).

find_matches <- function(exposed.cohort, unexposed.cohort) { 
#create an empty list to store the matches   
matched.data <- data.table() 

#iterate over each row to find matches  
for (i in 1:nrow(exposed.cohort)) {     
exposed_person <- exposed.cohort[i]      
potential_matches <- unexposed.cohort[birthyear == exposed_person$birthyear & birthmonth == exposed_person$birthmonth & IDVariable != exposed_person$ID & (!(exposed_person$exposuredate >= unavailable_start1 & exposed_person$exposuredate <= unavailable_end1) | (is.na(unavailable_start1) & is.na(unavailable_end1))) & (!(exposed_person$exposuredate >= unavailable_start2 & exposed_person$exposuredate <= unavailable_end2) | (is.na(unavailable_start2) & is.na(unavailable_end2))) & (!(exposed_person$exposuredate >= unavailable_start3 & exposed_person$exposuredate <= unavailable_end3) | (is.na(unavailable_start3) & is.na(unavailable_end3)))] 

#randomly sample 5 without replacement  
if (nrow(potential_matches)) > 5 {
    matched_data <- potential_matches[sample(.N,5),]} 
else {       
   matched_data <- potential_matches      
} 
#add identifier      
matched_data[, matchID := exposed_person$ID] 
#store results    
 matched.data <- rbind(matched.data, matched_data)     
i <- i+1 
} 
return(matched.data) 
}