I've used the store-bought scent with my last beagle when it was 3-6 months old.
But the best answer is Swampsinger's - if it's from a good hound breeding, it will learn on its own.
All it needs is practice time in the bush.
I think that's what the beagle trailers do - just run the dogs where there are rabbits and they'll figure it out.
If you need to teach your dog what a rabbit smells like - it's going to be a long road.