df <- data.frame(a=1:12, b=13:24 );
df;
##     a  b
## 1   1 13
## 2   2 14
## 3   3 15
## 4   4 16
## 5   5 17
## 6   6 18
## 7   7 19
## 8   8 20
## 9   9 21
## 10 10 22
## 11 11 23
## 12 12 24
n <- 5;
aggregate(df, list(rep(1:(nrow(df) %/% n + 1), each = n, len = nrow(df))), mean)[-1];
##      a    b
## 1  3.0 15.0
## 2  8.0 20.0
## 3 11.5 23.5