Because they aren't actually going to really fight the dragon, don't make it a young dragon - bring on a real beastie! I'd personally recommend a dragon turtle or some other kind of sea creature. Nothing scarier than danger from the deeps!
The best DM I ever played for did something like that to open a campaign - had an encounter with a huge red dragon to open the game...
For each PC, we had a quick mini encounter that he called our "James Bond" opening - a tough individual encounter that ran quickly. At the end of each mini encounter, he ended with something like, "as the last bandit runs off (or, after you finish off the orc, etc), the forest grows quiet and suddenly it gets dark out. You look up and see a huge dragon has temporarily eclipsed the sun as it heads due west towards Little Town." He filled it out with much better flavor text than that...but, each PC saw it flying towards the town from a different direction.
Of course, each person followed the dragon, where it had just finished annihilating the home of the party wizard's mentor, mentor inside.
The mostly unrelated PCs - all first level - now have a gargantuan red dragon where they meet for the first time.
For some unknown reason (at the time), the dragon then gave the party a strange look, snarled something like, "it's not possible" and then left. At that stage, it likely could have killed us while blindfolded and with both claws tied behind its back...but, it made for what I thought was a unique opening scenario that got the party together in an unusual manner.
(it's been like 10 years since that adventure, so my memory is hazy...)