How old was Avril when she started writing music? What was her 1st song???

Avril started writing her own songs and playing the guitar in her early teens.

The first song that Avril ever wrote was called "Can't Stop Thinking About You". It's about a teenage crush, which she described as "cheesy cute". The song hasn't been released.
Apparently a song called Darlin' was another of Avril's first songs. It might be appearing on her 4th album due out soon!

Source: link

Hope that helped :)