How old should a plant be when I transfer it outside?

I know some people start by seed outside, but what is the optimal (that it would have a good chance) age for it to be transferred outside in your opinion? I'm thinking that height is more important than age? I'm LSTing so its hard for height reliable..

I'm growing an indica dominant hybrid that takes 7-9 weeks to flower btw.

 

Thanks again! Looks like its about time to put my baby outside then. Gotta set the chicken wire up!