Использование пакета forcats
от Tidyverse
library(forcats)
ab <- data.frame(age = factor(levels = c("18-25", "26-30", "31-35", "36-40", "41-45", "46-50", "51-55", "56-60", "61-65", "66-70", "71-75", "Above 46")))
ab$age <- fct_collapse(ab$age,
"Above 46" = c("46-50", "51-55", "56-60", "61-65", "66-70", "71-75", "Above 46"))