id  name    year    position
1   Jane    1990    Sales
1   Jane    1991    Sales
1   Jane    1992    Sales
1   Jane    1993    Boss
1   Jane    1994    CEO
2   Tom     1978    HR
2   Tom     1979    Sales
2   Tom     1980    PR
2   Tom     1981    Boss
3   Jim     1981    Sales
3   Jim     1982    Sales
3   Jim     1983    PR

id   name    year    position
    1   Jane    1992    Sales
    1   Jane    1993    Boss
    1   Jane    1994    CEO
    2   Tom     1978    HR
    2   Tom     1979    Sales
    2   Tom     1980    PR
    2   Tom     1981    Boss
    3   Jim     1982    Sales
    3   Jim     1983    PR

new<-ddply(df, df$position=="Sales", function(df) return(df[df$year==max(df$year),]))

ddply(df, .(id, name, position), summarize, year = max(year))

arrange(ddply(df, .(id, name, position), summarize, year = max(year)), id, year)

library(dplyr)
df %.% group_by(id, name, position) %.% summarise(year=max(year)) %.% arrange(id, year)