The 18"x18"x18" is a little on the large size for a 6 month old baby, it would really depend on the baby at this point I would say. The thing to watch out for would be how well it adapts to the new surroundings.
I've had some pretty aggressive yearlings that I've moved up into full size 222 Vision enclosures that did EXTREMELY well!! OTOH, I've got a male that is going on 4 now, average size that everytime I upsize him from his tub that is about 18x15x12 he goes off feed and into hiding. He just doesn't "feel secure" it seems when he's got the space. He's incredibly beautiful too and I'd sure like to put him in a nice display cage!!
The good thing about slightly larger is that they can grow into them too.... I'd set him/her up, give it plenty of quiet time, you can cover the sides of the enclosure so that it doesn't feel so exposed to possible threat. If the baby perches well and continues to eat on a regular basis, you have the temps and humidity dialed in, the actual size of the cage doesn't really matter as much imo.