Isn't age a factor, guys?
I mean, I'm not going to turn round to some Dad and say "Your four year-old should be out there on the road!" but yes, if they're old enough, teens and up let's say, then they should be on the road. Newcomers, I'm easy with - if your mum takes up cycling to work after 20-odd years then good for her, the roads can be a dangerous place, so use the pavement if you wish - at the end of the day it's a safety thing.
Getting kids to ride young and ride sensible is key for confidence-building (not on their own at first, I'm not talking about ABANDONING them out there!) and cycling on the roads is great for building road-sense before you're old enough to drive. Driving a car becomes a lot easier if you're already familiar with road signs, judging distance and adapting to road conditions; we may all think we have this kind of common sense in buckets but there's no substitute for experience.
Ideal scenario: bike lanes THAT ARE CONNECTED and THAT GO SOMEWHERE USEFUL so that I can actually get to places!(Copenhagen, anyone? Brilliant little set-up) Otherwise, heck, if you're concerned, ride on the pavement, if you're confident, ride on the road.
(House tip: I've found that motorists give you a wider birth if you look less 'cyclist', i.e. if you're out in the old lycra and cleats they're happy to hug close to you 'cus they reckon you'll manage - tossers)