Kumquat Trees live from 40 to 50 years on average but some Kumquat Trees live much longer and possibly up to 100 years or even longer.
When you plant a Kumquat Tree it can take around 10 to 15 years for the Kumquat Tree to bear Kumquat Fruits so if you plant a Kumquat Tree and want Kumquats from then then be prepared to wait for several years.
However after all that time of waiting for the Kumquat Trees to produce Kumquats it will continue to produce Kumquats for many years to come.
That goes for other fruit trees as well as they can take 10 to 15 years to produce fruit.
So you won't get fruit or Kumquats from a Kumquat or Fruit tree overnight as it will take awhile.