So I got some friends into LoL before I knew about the Refer a Friend feature. Is it possible to still get points for them, or is the only way if someone creates an account through it?
No. As you probably expected, once an account has been created, it cannot state a referrer, regardless of it's level or age.
The only chance to refer a player comes directly after preliminary signup, while still in browser. After inputting information such as 'Account Name', 'Email Address' and such, you'll be taken to the 'Client Download Screen' seen below.
As well as the option of downloading the client, the option to state a referrer is given at the bottom of the screen. After navigating away from this page, you lose your chance to input a referral.