Select row with most recent date by group

Question

I have a data frame in R where the rows represent events, and one column is the date of the event. The thing the event is happening to is described by an ID column. So for e

Answer 1

I've never processed any data in R without plyr!

library(plyr)
ddply(df, .(ID), summarize, most_recent = max(as.Date(date, '%m/%d/%Y')))

   ID most_recent
1  1  2001-03-14
2  2  2008-02-01
3  3  2011-08-22