Just what is that black rock that Morgan puts in Arthur's crib?
Pure evil. *ahem* ... No, seriously, it seems to have been something to taint his heart with darkness so that he'd be flawed and Merlin would never have his pure and completely just king.
In the novelizations, it put a curse on Arthur, specifically resulting in his recklessness as an adult that led to him chasing after the Grail, and opening the door for Guinevere's affair with Lancelot.
If I could magically rejuvenate myself and my sweetheart, I surely wouldn't settle for middle-age, but return at least to our young adulthood.
Technically, Merlin was in young adulthood during most of Part 1. It's just that Sam Neill always looks about 45-ish.
But Nimueh was changing over the story time. She transformed back into her middle version, not into the one when they met.
He turned them back into the time where they loved each other best. Besides, he pretty much said it was basically his last hurrah of magic... he might not have had enough to turn them back into near-teenagers.
"Magic can't create love." Except for when Mab does it twice. Merlin isn't a very knowledgeable wizard.
It can create lust, infatuation and obsession, but not love. Those things can lead to love, but aren't in and of themselves.