As flying adults, dragonflies live only a few weeks, but if you include their underwater stage as nymph, their lives span from 1 summer up to 3 years. Adult flying dragonflies eat other smaller flying insects, esp. mosquitoes and gnats.
Nymphs eat other underwater life forms such as mosquito larva.
Larger nymphs can and will eat pollywogs and small fish. Adult dragonflies die of old age as winter approaches, but nymphs from spring, summer and fall laid eggs are still maturing underwater. On Earth there are over 5,400 species of dragonflies and damselflies. In the USA and Canada there are 435 species altogether. Since they have a mouth, they can bite, but they bite prey, not humans, unless handled roughly.