Hi there, as a junior F/O for Sabena, I will try to solve this question for you.
To start as a F/O at Sabena, you need to have a valid Begian (or JAA -> European) CPL/ME/IFR license with a frozen ATPL.
To obtain a valid JAA CPL/ME/IFR you have to be 18 years old.
A frozen ATPL means you have done the theory, but still lacks the required 2000 hrs. of flying experience needed under Belgian law to receive the ATPL (as from the age of 21 years.)
So, the required age to become a F/O on B737, BAe146 or A320 is 18 years at Sabena!
I can assure you there are quite some guys (and girls) from 20 or 21 flying in the right seat of these planes, including myself (21 years old).
As pilots, we in Europe have the benefit that mayor airlines do not require huge flying experiences like in the United States. Since flying a big jet is very different then flying VFR in a Cessna, there is no real benifit in having 3000 or 4000 hrs on General Aviation machines. I really do not know why Delta, American, United etc. don't want to hire pilots having (only) 300 to 400 hrs in their logbook...
All mayor companies in Europe (Lufthansa, KLM, Sabena, British Airways, ...) do it. You can hardly say that they have an unsafe reputation, can you?