The day after my 16th birthday, because my birthday was on a Sunday that year.
In the 70's, gas was cheap, cars were cheap, and teenagers either had their own car or it was a lot more common to borrow Mom or Dad's car. My mom and dad each had their own car, and when it was time to get a new one for Mom, my sister and I "inherited" her old one. I mean, we didn't own it per se, but the family had three cars from that point forward, and us kids had custody of the third one, eventually including my younger sister as well. We put in the gas, paid for the oil changes, and paid our own car insurance (which was also hella cheaper back then).