To establish a global policy such that passwords never expire, set default_password_lifetime to 0:
  default_password_lifetime value is 0, which disables automatic password expiration


[mysqld]
default_password_lifetime=0


SELECT
   user,
   host,
   password_last_changed,
   password_lifetime,
   password_expired
 FROM mysql.user
 WHERE user = 'test_exp'\G

 ALTER USER 'script'@'localhost' PASSWORD EXPIRE NEVER
 The password_lifetime value was updated from NULL to 0, indicating that the password will never expire (NULL means, “use the server default”).