It is a 2 part skill. The first part reduces your Psi score by the amount of ability reduction from aging and is permanent. If you Psi score is high enough, you stop aging without affecting your Psi score.
I had previously read it as you go through the first part and in time reach the second part. But if you take too long, your Psi will be too low to make use of the second part. Reading it again, I see that if your Psi score is high enough, you stop aging at the cost of your Psi score, and if it's higher, you stop aging with no cost.
The text says it's a Droyne specific ability, like Invisibility, but if it's hard to get, most Psionic users might not know of the ability, and the few who do might not realize they are aging slower. I mean, why meditate for an ability no one knows exists?
I kind of like the idea of making a skill check, so that it's more like the time you spend meditating or training affects your aging slowly instead of immediately.