Select minimum value from multiple rows

I’m working with a dataset where a number of subjects had follow-up examinations over a period of time. The age at every examination is recorded:

subject <- c("subject1", "subject1", "subject2", "subject2", "subject2", "subject3", "subject3")
ageAtExam <- c(50, 51, 30, 32, 35, 40, 45)
df <- data.frame(subject, ageAtExam)

> df
   subject ageAtExam
1 subject1        50
2 subject1        51
3 subject2        30
4 subject2        32
5 subject2        35
6 subject3        40
7 subject3        45

I’d like to create a new column where the age at the first examination (i.e. the minimum value in ageAtExam for every subject) is given:

   subject ageAtExam ageAtFirstExam
1 subject1        50             50
2 subject1        51             50
3 subject2        30             30
4 subject2        32             30
5 subject2        35             30
6 subject3        40             40
7 subject3        45             40

I’m grateful for any suggestions, thanks!

>Solution :

Using ave

> ave(df$ageAtExam,df$subject,FUN=min)
[1] 50 50 30 30 30 40 40