The key here is that the pharmacist used the results of a study of people aged 65-81 with insomnia to support the claim that the pineal gland produces less melatonin as it ages. This constitutes an unrepresentative sample because (1) it is not looking at anyone younger than 65, so we cannot comment on how melatonin production changes with age, and (2) it is only looking at people suffering from insomnia, and this group may be an exception to the normal operation of melatonin production.
Does that make sense? Please let us know if you have any other questions!