When to start backlinking to my website?

There is no specific time to start it, You have sufficient content and Domain date to start.

So you can start it from now.

You can start building links. But the issue is quality backlinks.

People who maintain websites seriously, no one wants to link back your website with 5/10 pages until you pay for the link.
All webmaster or sites owner want to link only authority or a quality website which are info rich and authentic.

Hope I can make sense.