you are full crazy

A teacher was taking a math's class, suddenly the principal came in for inspection and asked a question to the class.

"If a train is moving at the rate of 30km/h then what is my age now?"

Nobody including the teacher was able to answer. After a while a dull fellow raised his hand and said "50 sir". The answer was surprisingly correct.

"Absolutely Right Little Genius" Replied the Principal" But how could you calculate so accurately?, what was the formula behind?".

The boy said,"It's Simple! We have a neighbor of age 25 and the doctor says he is half crazy"