Why does summarize or mutate not work with group_by when I load `plyr` after `dplyr`?

The problem here is that you are loading dplyr first and then plyr, so plyr's function summarise is masking dplyr's function summarise. When that happens you get this warning:

library(plyr)
    Loading required package: plyr
------------------------------------------------------------------------------------------
You have loaded plyr after dplyr - this is likely to cause problems.
If you need functions from both plyr and dplyr, please load plyr first, then dplyr:
library(plyr); library(dplyr)
------------------------------------------------------------------------------------------

Attaching package: ‘plyr’

The following objects are masked from ‘package:dplyr’:

    arrange, desc, failwith, id, mutate, summarise, summarize

So in order for your code to work, either detach plyr detach(package:plyr) or restart R and load plyr first and then dplyr (or load only dplyr):

library(dplyr)
dfx %>% group_by(group, sex) %>% 
  summarise(mean = round(mean(age), 2), sd = round(sd(age), 2))
Source: local data frame [6 x 4]
Groups: group

  group sex  mean    sd
1     A   F 41.51  8.24
2     A   M 32.23 11.85
3     B   F 38.79 11.93
4     B   M 31.00  7.92
5     C   F 24.97  7.46
6     C   M 36.17  9.11

Or you can explicitly call dplyr's summarise in your code, so the right function will be called no matter how you load the packages:

dfx %>% group_by(group, sex) %>% 
  dplyr::summarise(mean = round(mean(age), 2), sd = round(sd(age), 2))

Your code is calling plyr::summarise instead of dplyr::summarise due to the order in which you have loaded "plyr" and "dplyr".

Demo:

library(dplyr) ## I'm guessing this is the order you loaded
library(plyr)
dfx %>% group_by(group, sex) %>% 
  summarise(mean = round(mean(age), 2), sd = round(sd(age), 2))
#    mean   sd
# 1 36.88 9.76
dfx %>% group_by(group, sex) %>% 
  dplyr::summarise(mean = round(mean(age), 2), sd = round(sd(age), 2))
# Source: local data frame [6 x 4]
# Groups: group
# 
#   group sex  mean    sd
# 1     A   F 32.17  6.30
# 2     A   M 30.98  7.37
# 3     B   F 38.20  7.67
# 4     B   M 33.12 12.24
# 5     C   F 43.91 10.31
# 6     C   M 47.53  8.25