Do humans learn to distinguish different senses?
Mixing up input from different senses is actually a known neurological phenomenon called synesthesia. Synesthetes (ie. adults experiencing synesthesia) might see numbers in colors for example. Some may even "taste" colors as you say.

Now do all infants have synesthesia before some stage in development is another question. It is actually a rather old hypothesis called "neonatal synesthesia". One piece of evidence comes from this study which tested shape/color association in infants. I do not see any other study concerning this hypothesis though. I guess the main reason might be the difficulty to develop an objective experimental design on small infants who hardly communicate...