How can you tell the year of the bike (mountain)??

Question

I have 3 bikes that were given to me, I know one of them is somewhere 1980's, and couple of them are in 1990's but how can you tell the exact year? I was told there's a specific # you can tell by?? Any tips how to tell??

1980's 26" Roadmaster - 10 speed

1990's 26" Magna - 15 speed

1990's 26" Something - 18 speed

Thanks!!

Answer 1

There should be a serial number stamped on the bottom of the bottom bracket. The serial number usually contains the month and year built. Not sure why this matters as these bikes all together are worth <$100.

Answer 2

yea mr you are right