not null unable to use in mysql

Question

I am learning mysql myself but I faced this problem with my server when I want to create a simple table.

CREATE TABLE customers (first_name 'NOT NULL',last_name 'NOT NULL',age);

Error:

ERROR 1064 (42000): You have an error in your SQL syntax; check the 
manual that corresponds to your MySQL server version for the right syntax 
to use near ''NOT NULL',last_name 'NOT NULL',age)' at line 1

I have no idea what is the problem as this command was learnt from website, it should have been fine to use the NOT NULL.

my sql server is 8.0.22