Is stimulus generalization acquired due to inability to discriminate stimuli?
If you think of the stimulus being represented by a distributed set of features (Tanifuji et al., 2001), then I would not say it was a failure to discriminate. During the conditioned response training for Albert, all features were present as the stimuli. It wasn't that he couldn't discriminate the features, just that each of the features were trained as conditioned stimuli.  

I suspect that an adult would think of the rat symbolically, and the conditioning would be done on that mental rat symbol. The individual features would likely not produce fear. Children of Albert's age are not thought to have yet developed symbolic thought (DeLoache, 1987). So, the conditioning would be done on the visual components themselves.

References

http://anon.cs.rochester.edu/users/faculty/dana/tanifuji.pdf
http://www.sciencemag.org/content/238/4833/1556.short