I feel like this is question somewhere between, “Does intelligent like exist outside of Earth?” And “How many angels can dance on the head of a pin?”
There’s really no definition of the “real value” of anything past “what the market decides” so, even if we were omniscient, we still might not be able to provide a truly factual answer.
But if you and I agreed to some definition then we’d still probably have a real hard time, short of omniscience, since value would still come down to factors like knowing the minds of everyone who works at the business and the sorts of decisions they would be liable to make in the future.
Ignoring all that, though, we could say things like that any time the market is undervalued and quickly rises back up to its “correct” value, then that wouldn’t be a bubble despite being a quick rise.
A first pass search for such a moment, we might do something like assume that everything should just grow at a static 7% a year (or whatever percentage looks to create a center line through history) and then then look at when the market average was above and below the line. A better search might compare to the GDP.
The chart here would imply that the period from 1995-1996 was a positive correction and the period from 1996-2000 was a bubble. Likewise, 2003-2013 was a positive correction.
But, the chart would also say that we were in “bubble” territory from, effectively, 1954 to 1974 (excluding a brief moment of correction in 1970).
I’m not sure that it makes sense to tell people to not invest for multiple decades at a time. If it was 1962, and we were in +2 std dev territory, then I’d probably advise a 20 year old to throw their money in the market but to lean more towards stocks that pay dividends, and to reinvest the dividends, but to watch for drops and pick up growth stocks if ever things are more in line with the expected market valuation. BUT, talking to a 90 year old, I’d advise strong caution because who knows if the end of the bubble will come in 2 years or 20? I’d probably suggest that they buy into REITs or something that just keeps up with inflation and pays out a bit extra as spending money.
Compound growth just isn’t so strong over short runs of time. At 90, even if things don’t implode, how much are you really likely to grow things before the sky opens up to take you away from the world? By your age, you basically have what you have. If it wasn’t quite as much as you hoped for…well, that’s just what fate and your choices through life left you with. It’s probably not going to get better and the risk of losing even that isn’t to be messed with.