How can I use arel in Rails to perform a simple OR clause

I have a table:

User(name, age) 

I would like to select all the users that are less than 18 years old or older than 80.

I know that I can achieve this the old way using the following query:

User.where("age < ? or age > ?", 20, 80).to_a

But I would like to build the expression dynamically, so I'm interested in how I can do this using arel.