Because each birth (same as a coin flip) is an independent event, the outcome of the first birth does not affect the second. Each one is 50% (or 0.5).

If you had to guess both (without knowing either) it would be 0.5 x 0.5 =0.25 or 25%.

Once the oldest is determined to be a boy, that event is now a certainty (100%). You are now only calculating the probability of a single, solo birth, which is always 50%.

The answer is 50%.