While we don't know precisely when my cat was born, today is his "official birthday." He is thirteen years old today.
This is roughly equivalent to being a 70 year old human. Indoor cats typically live 14 to 20 years.
Notably, the relationship between cat years and human years is not linear, cats reach adolescence much sooner than humans, i.e. in about a year they are equivalent to a fifteen year old human, and by the time that they are about three calendar years old when they are roughly equivalent to a twenty-five year old human, start aging at a rate of about four cat years to one calendar year.