When to upgrade enclosure?

I have two females that are both 16 grams. I'm currently keeping them in separate 12x15x12 tubs. At what weight would it be ok to move them into my 18x18x24 exo terra?

Forgot to add they are crested geckos

I upgrade mine to that size at around 20 grams or so.

I usually move my holdbacks to a bigger cage (17x17x23) when they are between 15-20 grams.

Thanks for the help :)