DNA replication is imperfect, but it has to happen whenever a cell divides. So every time a cell splits into 2 daughter cells, the daughters have a few tiny differences in their DNA. The copying and proofreading mechanisms are highly effective - the error rate is as low as one in 10 billion - but cells divide regularly throughout your life, so plenty of mistakes start to build up. Soon enough it makes a difference to your health and strength.
But families and species don't age, despite individuals doing so, because there is a method of periodically getting rid of unfavourable copying errors in DNA - sexual reproduction. Sperm and egg cells are produced in vast numbers, and then any which show the slightest sign of being substandard are killed. A very large proportion of newly fertilized embryos miscarry as a result of serious errors not caught at the sperm-or-egg stage, and that also keeps copying errors out of the surviving next generation.
Plants and animals which don't put any energy into looking after their offspring are dead ends, from the point of view of DNA replication, so there is no strong pressure for DNA replication to continue.
I don't think your DNA mutation rate depends on how much you sin - apart from smoking, of course
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