I am not sure exactly what year it would be, but sometime in the 80s RPGs went from being a hobby cottage industry to a viable publishing category (even if a small one). That happened in large part due to the ubiquity of desktop publishing. That is the start of the Silver Age, IMO. I think the d20 glut and crash, and how it fueled a lot of indie games from the Forge, is probably another line of demarcation. I am less confident about calling the start of the "Modern Age." A lot of people would align it with 5E but I am not so sure. I might align it with the rise of Actual plays, which was parallel to 5E but not because of it (although the reverse might be true).I invite you, once again, to explain your criteria or definition.
I don't think golden age, silver age, etc. apply to RPGs in general, but I'd be interested to hear a case made. I think I've seen James Maliszewski apply them to different periods of TSR's history specifically.
When I first got onto Dragonsfoot 2nd ed AD&D was definitely not considered Old School D&D by that crew, but the consensus has shifted in the last 15 years or so from what I can tell.
I don't think it's useful or accurate to confine our use of the term Old School in referring to RPGs to, say, the first 5 years/10% of their existence.