Hi Sabine
I would say: It is nearly full grown - I have noticet, that the max larvalength is 15 mm, so 13 mm must be near to "full grown".
Additional it is absolutely in the end of the official larvatime, in fact a little late, but the summer yesteryear was strange and everything
was late due to the very rainy spring (in Denmark)

I have to add, that I have not breeded these larvae. I never do. - First of all I am a photography

I hope, that this is answer enough - or you just ask again.

Greetings - Kjeld