Displaying a greater than or equal sign

An alternative to using expressions is Unicode characters, in this case Unicode Character 'GREATER-THAN OR EQUAL TO' (U+2265). Copying @mnel's example

.d <- data.frame(a = letters[1:6], y = 1:6)

ggplot(.d, aes(x=a,y=y)) + geom_point() + 
    scale_x_discrete(labels = c(letters[1:5], "\u2265 80"))

Unicode is a good alternative if you have trouble remembering the complicated expression syntax or if you need linebreaks, which expressions don't allow. As a downside, whether specific Unicode characters work at all depends on your graphics device and font of choice.

You can pass an expression (including phantom(...) to fake a leading >= within the label argument to scale_x_discrete(...)

for example

 .d <- data.frame(a = letters[1:6], y = 1:6)

 ggplot(.d, aes(x=a,y=y)) + geom_point() + 
    scale_x_discrete(labels = c(letters[1:5], expression(phantom(x) >=80))

See ?plotmath for more details on creating mathematical expressions and this related SO question and answer

plot(5, ylab=expression("T ">="5"))

You can use

expression("">=80)

So your full axis label like would look like:

scale_x_discrete(labels=c("0-29","30-49","50-64","65-79",expression("">=80),"All")) +

I have had trouble exporting plots when using unicode, but the expression function is more consistent.