Passing string as column name in lag()

I need to use the function lag() to obtain a new column in a data frame, but within a loop. Therefore, the column names are given as a string. When I do it, I get only NAs. See below a very simplified and reproducible example. I removed the loop because it is not relevant.

df <- 
  data.frame(age = c(1, 2, 3),
             value_2010 = c(10, 20, 30))

new_col_value <- "value_2010"
new_col_name <- "value_lag"

df_new_column <-
  df%>%
  mutate({{new_col_name}} := lag({{new_col_value}}))

This is what I want

And this is what I get

>Solution :

You may use !! instead of {{}}

df %>%
  mutate(!!new_col_name := lag(!!as.symbol(new_col_value)))

  age value_2010 value_lag
1   1         10        NA
2   2         20        10
3   3         30        20