does alcohol go bad?

[gamblore99]

A friend is moving. He has a about 20 bottles of alcohol he wants to sell me for 150 canadian. The bottles are about 10-20 years old and are unopened. (he got them from parents who got them as gifts over time. No one in his family drinks)

Some of the stuff is very nice. Most is whiskey, scotch, rum, but there is other stuff like tia maria and baileys.

Anyone know if sitting on his shelf for 10-20 years affect the stuff?