Unless you used unsafe water or honey, it is safe to drink at any time. The alcohol in the mead effectively keeps it safe for consumption. The fact that it smells good is a good sign and even with some off-flavours everything should be on the safe side.
Pungent smells, mould and no alcohol at all, however, are signs that something is wrong.
Generally, fermented drinks are safer than untreated (and possibly contaminated) water because of their alcohol and acid content. That doesn't necessarily mean they are healthier though.
Depending on the yeast strains involved, the length of the fermentation and what is fermented, fermented drinks may contain substances that make you feel unpleasant (hangover).
Most fermented drinks are ready to drink once the yeast has settled (and the drink loses its yeasty taste). They continue fermenting albeit at a much slower pace. This means
their flavour changes over time, usually getting more acidic while losing some or all of their sweetness. In other words, it is ready to drink when it tastes right.
To prevent further fermentation, you can store your mead in a fridge or a very cool cellar which slows down the yeast significantly.