My father has practically no sence of smell and I have a fairly poor one, so genetics may in part be involved. However, mine has actually got better with age, which is interesting to me. I used to have many many colds as a child, which I think means I never learnt to smell as a child, and hence as I got older (and my colds became less frequent) I had to learn to smell at a later age. I used to mis-recognise smells all the time as c. 10yo (prawn cocktail crisps were the most common: for some reason I could smell them instead of any other strong smell for some years. Eventually that stopped (thank goodness)).

In reference to Q.3: I think your teacher could well have been right. I have read about infants who became blind in one eye because they wore an eyepatch for a few weeks during a crucial part of development, and those neurons which were therefore unstimulated, died off. I've also heard of this happeing with children who for some reason had an ear blocked (treatment for an ear infection or similar) and became deaf. This is very common in infants, since far more neurons are created than can be sustained by the brain, so anything that doesn't get stimulated dies off. Possibly the same thing happened to you: it's certainly the cause I cite for my impaired sence of smell.