It's not that these snakes necessarily lose arboreal tendencies as they age. Rather I don't think keepers give them cage furniture condusive to a large snake perching. We give them twigs and when they stop using them say it's because older snakes don't perch.

Most snakes when given the right sized perches will continue to perch well into old age. I have found that the older larger snakes actually prefer a y type branch to perch on rather than a single dowel/branch.

That being said, there's nothing wrong with keeping them "terrestrial"...