Is 40 year’s old still considered “young” ?

Yes, 40 is still young because you can double it to 80.....fifty can be doubled to 100...but 60 can't be doubled.  Source Dustin Hoffman

Forty YEARS old.  

Well that depends - considered young by whom, and in which context? Obviously to a 6 year old, 40 would probably seem very old. A 20 year old college student might think a 40yo professor seems pretty young compared to other professors who are older, but when they think of dating a 40 year old compared to someone their own age, then it does seem old (though some people are drawn to older partners).

Well, I wouldn't say 40 is young, even 30s aren't that young really, but it's still on the younger side of life. Your 40s are the beginning of middle age, but  it's still the prime of your life in many ways, and with people living much longer now on average, ageing isn't perceived the same as it used to be. Just remember, James Bond is in his late 30s to mid 40s. When Roger Moore played Bond, he was between the ages of 46 and 58! Daniel Craig is now 52 and has recently filmed his latest Bond movie.

Quite honestly, you are as young as you feel and if you feel young, then great. Unfortunately a lot of society these days is judgmental particularly young people and although 40 might seem like the new 30 to you. The reality is that young people and when I say young people I mean mostly millennials and Z generation think and believe that 30 years old is over the hill, lol. My advice is, live your life and don’t worry about the rest, including opinions.

No.  You're already in middle age, based on life expectancy.  I just hope you're already planning your retirement.

I suppose to a 90 year old it would be

You are as young as you feel.

Yes, at least by everybody over 50

Absolutely true.  However, a lot of it has to do with the person.  If you take care of yourself through exercise and healthy eating and if you stimulate your mind by trying to learn new ideas to replace your outdated ones, and if you keep your sense of humor, you will keep  your youth.

It's relative. An old person is someone 10 years or more your senior. A young one is at least 10 years your junior.