If A man was born in 1955 and today is his 18th birthday how is that possible?

Answer 1

Since 1955 was not a leap year, the only possibility is that he had learned to travel very very fast - at approx 95.5% the speed of light. Following the time-dilation effect of travelling at high speeds, this would do it.

Answer 2

Because "today" happens to be in the year 1973.

Answer 3

That means that "today" is not 2016, but a year that is 18 years after 1955.