I have a spark plug that I bought when I first planned on working on the bike I have now, I been saying it was 35 maybe 40 years ago. I do the math, and that may not be close either. 

Is there any way to date a plug by either the markings on the box or numbers stamped on the plug?

  

I bought the plug along with a few other parts that simply sat in a bag for 35 plus years until I recently installed.

I know its trivial, but it is so long ago, I can't remember how old I was when I put the bike up for the time in hopes of getting it going back then, although it took decades to get to it. It is obvious it is a 49 year old bike, I bought it used, I was in my 20s, maybe early 30s????

Well we know its at least 35-40. They get bertter with age, like wine. Probably made in China now. I've probably got some used ones that old but never thought about it till now. Be interesting to compare with others old plugs put aside for re-using in desperation oneday.