The prof's age

Dear All
Its just for fun...

Professsor: Given the age of the light as 3 x 10^8 m/s and the density of water as 1km/m^3, what's my age?

Student: I'd say about -- 46.

Professor: Excellent! How did you know that?

Student: You see, sir, my brother's 23 and he's only half-mad.