How is the age of my haplogroup calculated?

The age estimates when the ancestor in whom the variant defining your haplogroup first appeared lived. It is calculated using a "molecular clock": the genetic variants that have accumulated since then in the lineages of that haplogroup are counted and compared with the average rate at which new variants arise, calibrated using ancient DNA and known genealogies. This is why it is an estimate with a margin of error, often spanning thousands of years. All your ancestors along that lineage since that time belong to your haplogroup; variants that arose later define your subhaplogroups.