see the chassis no you might be able to make out from it the month and year of manufacture
the no is likely to contain a serial no , may be an alphabet linked with Serial no , the month and the year of manufacture.
the no will be found on th tubular frame generally below the fuel tank
alternately look for engine no on the central casing of the engine (not on the removable outer casings ) ,again most likely to be below the fuel tank on right side
in case the vehicle is not very old and assuming the tyres have not been changed u can find the year of manufacture of the tyres and guess the nearby age of the bike.