I've not only done that, but run an introductory adventure with the PCs as children that know each other and play together. I first did this with a group of new players with the idea that I'd run a sort of "whiffle ball" adventure to let them learn the mechanics of playing the game.
The results were such that, as often as possible, I try to do this with each new campaign now if time permits. The party cohesion was better, there were strong connections between the characters that acted as a strong basis for excellent role-playing, and the overall narrative felt much deeper through the rest of the campaign.
The introductory adventures I run tend to be something like a "Three Investigators" mystery of some sort, good for about 5-6 hours of play time. I try to make sure the characters have access to some special resources for transport, items, and such that allows them some freedom of action. Sometimes I'll also introduce some characters that I've got plans for in the adult adventure as well--you can never introduce a villain too early.
For Traveller, I modify the characteristics for whatever age the characters will be. Strength and Edu are obvious. I give them some homeworld skills at 0 that seem logical from what they'd have in normal chargen.
Basically, I highly encourage having "pre-campaign" or young adventures.
I've had my group request starting characters at 10-12 years old and running the campaign continuously through to adulthood. I haven't done that yet, but the idea is appealing. Presently I usually can't run a single campaign long enough to have that much material in one campaign. I usually plan about 50-60 hours of play into each campaign. If I ever get a group I can pretty well expect to run an open-ended game again, then I think it'd be fun to start the characters out young and see what happens.