I've seen this argument offered before (i.e. Arnold starting at a young age) and it rarely makes sense to me. The guys who started really young and went on to compete made a decision that this was going to be their profession. Body building was more than a recreational or lifestyle choice - it was a career. They were driven to become professional BBs and were willing to do what ever it took to WIN.
The majority of people on this board aren't using steroids to compete professionally. Yes, some chose to compete but even that level is often not at a professional card level.
Most people clearly have no clue the level of commitment these professionals make to BB. The amount of food intake and anabolic use at the professional level would blow most people's minds.
Frank McGrath, shown below, who I've met and chatted with a few times, takes extreme doses of gear at levels that are dangerously unhealthy, but at the pro level it's about winning - health and future consequences are secondary.
Frank often cycles 2-3 grams of test a week during MAINTENANCE periods between competitions. More when he competes! His caloric intake ranges from 3000-6500 depending on where he's at in his contest preparation.
Antoine Vaillant, another pro IFBB out of Montreal who I've met several times also takes insane doses of gear on and off cycle - doses that would likely kill a horse!!! Lol
There are very few guys on this board that are ready to make the level of commitment the pros make. Very few guys here will or have given up their school, family, or jobs to make a career out of body building at a professional level.
So unless you are ready to commit to BB full time and pursue a career at the professional level, you really can't make comparisons with the pros and ask, "why not, they did it when they were young".