The groups both consist primarily of adults. I don't think we have had any younger players, but I know I would be ok with having teenagers (and I can't imagine my other GM would have a problem either). With the inherent themes that come along with Shadowrun (Violence, Crime, etc) I generally consider it a PG-13 game, so I do caution against bringing very young children into the fold early.
As far as character creation, the Missions game follows standard creation rules for the standardized play. The Missions FAQ (found
HERE) contains the list of limitations and also a description of what you can expect in terms of setting.
For my custom game, we used standard Priority creation, although if you are interested in one of the other creation methods, I would be open to it. The team is in the middle of a run at the moment, so I'm not sure how easy it will be to integrate a new player in this week. You're always welcome to observe, and I could also hand off an NPC that is helping out the group on their current mission.
Do you have a preference between playing Missions standard vs custom campaign?