Year of my bikes

Hello, I have 3 bikes that were given to me, and the previous owner couldn't really tell me the year of my bikes.

First bike is Roadmaster Citybike, 10 spd, ser# C70218393.

2nd bike is Huffy Stalker LX, 18 spd, ser# 93371huffy26661. 

3rd bike is Huffy Superia, 15 spd, ser# K6677--9928812H6652. 

 So can anyone tell me how you can identify the year of the bike manufacture in as I was told you can tell by serial #, is this true?

If so, please explain to me.  Thanks!!