Only for the lucky ones! In all seriousness, I think it's a matter of correlation more than causation. You can probably assert that over 90% of hard drug users have used marijuana, but the percentage of marijuana users who move to hard drugs is nowhere near that high.
The bottom line is that marijuana is much more accessible, so someone with an inclination to heroin may start with marijuana for reasons of availability and cost, while the casual pothead starts and ends with marijuana because that's what they like.
I would think that most Ferrari drivers started out with something slower and less expensive. Does that mean that the Geo Metro is a gateway car to Ferrari?
Yes.