A blue moon is two full moons in the same month. How many times will a blue moon occur in the average lifetime of say 70 years?
As months are not of fixed length it is only possible to calculate an average.
The average frequency of a blue moon can be calculated as follows. It is the period of time it would take for an extra synodic orbit of the moon to occur in a year. Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, then there are about 12.368 synodic months in a year. For this to add up to another full month would take 1/0.368 years. Thus it would take about 2.716 years, or 2 years, 8 months and 18 days for another blue moon to occur.
So for a 70 year life span the average number of blue Moons would be 70/2.716 or 25.77.
Be aware of the other kind of event also referred to as "a blue moon".
After some large volcanic events, with volcanic ash sent high into the stratosphere and circulating widely around the planet, the moon has appeared blueish in hue rather than the usual reddish, such as seen with total eclipses of the moon.
These "blue moon" events are very rare, and can not be predicted.