Due to gravitation time dilation, your upper body is technically older then your lower body due to Gravitational Time Dilation. Gravitational Time Dilation the effect gravity has on time. The lower the gravitational potential, the faster time is. Knowing this, because your dick has more gravitational potential then your head, your head is older then your dick (by like less then a millionth of a nanosecond but that doesn’t matter). So if you were sucking your own dick while you turned 18, your head would age first. This means your 18 year old head would be sucking you 17 year old dick.