It depands on the cell the DNA is in. For eg, if it’s in a cell with a short life span, it doesn’t get old. For something like bone cells, those last a long time, like a life time. Saying that when cells in eukaryotes divide the DNA replication is semi-conservative which means maybe there is DNA within animals which lasts from birth to death (I no longer study cell bio so I’m a bit vague on this topic). As for how long DNA has existed on Earth: just under 4 billion years.
