There are many 18 / 19 year old integrated scheme pilots in the right seats of B.737 and Airbus aircraft for BA, Titan, Ryanair to name but three companies.
I state integrated because to achieve this by 19 it is hard to complete by the modular route, due to the timescale involved, but not impossible.
I know of several 150 hour pilots ex integrated training who have achieved the right seat straight out of training having started training at 18 from zero flight time.
There is really no need to plod through as an instructor, or General Aviation pilot to achieve the airline goal in good times, but now as fuel costs bite and airlines lay off pilots the opportunities will deminish or cease to exist.
BA have a hiring freeze, U.K. Charter airlines are about to lay off or retire early up to 300 pilots.
Not to mention possible airline collapses in Europe over the winter 2008 / 9.
From a trainers point of view, its just as easy to train a cadet to line standard as a more experienced pilot who may have the extra hours but are less flexible to new SOP's and may have inherited some bad habits over the years.
On the plus side the more experienced co-pilot may better at handling and flying raw data than his cadet colleague, but any airline with a good training department will have a scheme in place to advance cadet's to a similar level whilst online.
Today we have many pilots who have only flown glass cockpits and have little or no other experience, but yet are first class operators of their equipment.
The old argument is do we count hours or sectors towards experience and I am NOT looking to open the debate here.
18 to 20 year old's in the right seat are certainly common in Europe, but remember they have to achieve the same standards as any other jet first officer to get into that seat and stay there, and it takes ability, maturity and (Yes) money to get there.