Most cows are slaughtered and killed for meat when they are around 36 months of age which is 3 years old.
The best quality meat from cows comes from cows 36 months old and under and most cows are fed so that they are fattened up to allow the cow to be killed for meat within 3 years of it's life.
A typical cows lifespan is around 15 to 20 years when they are not being slaughtered.
But most farms who raise cows for meat to be sold kill the cow for the meat within 3 years of it's life but some may go longer than 3 years.
You can still get good high quality meat from cows above 3 years old but the older the cow is the less quality the meat becomes.