I used the data you posted to make a data set with both "treat" and "placebo" values. Don't worry if how I did that is not clear.
I then show two ways to make a histogram of the two subsets. The data set is so small that the histogram is ugly but I hope it is enough to show you the method.
#invent data
library(dplyr, warn.conflicts = FALSE)
DF <- structure(list(time = c(213.4418689, 168.6752202, 268.2073257,
289.8671837, 272.5496501, 203.4556788),
age = c("21-35", "21-35","21-35", "21-35", "21-35", "21-35"),
group = c("treat", "treat", "treat", "treat", "treat", "treat"),
memory = c(7.196530159, 7.07164458, 7.572323115, 7.42767848, 7.736018388, 6.920243195)),
row.names = c(NA, -6L), class = c("spec_tbl_df", "tbl_df","tbl", "data.frame"),
spec = structure(list(cols = list(time = structure(list(),
class = c("collector_double", "collector")),
age = structure(list(), class = c("collector_character", "collector")),
group = structure(list(), class = c("collector_character", "collector")),
memory = structure(list(), class = c("collector_double", "collector"))),
default = structure(list(), class = c("collector_guess","collector")), skip = 1L),
class = "col_spec"))
DF2 <- DF %>% mutate(group = "placebo", memory = memory * 0.9)
AllDat <- rbind(DF, DF2)
#histogram
library(ggplot2)
ggplot(AllDat, aes(x = memory, fill = group)) +
geom_histogram(binwidth = 0.5, color = "white")

ggplot(AllDat, aes(x = memory, fill = group)) +
geom_histogram(binwidth = 0.5, color = "white", position = "dodge")

Created on 2021-01-17 by the reprex package (v0.3.0)