[run4yourlives]

The quantifier of 18 is not really accurate, although I agree with the general sentiment.

For many (most?) people, 18 is simply the start of the journey. That journey may be short or it may take decades, but the idea that you are somehow static after a certain age is certainly something coming from a very young mind.

[lifeisstillgood]

perhaps it was phrased wrongly - certainly there are convincing studies that one gets happier (by whatever metrics) as one progresses through the decades.

However if we have happy as a spectrum, I contend people are placed by the end of childhood on a point on that spectrum - some lucky ones far to the right, others not. One can move up the spectrum, but not everyone ends up crowded to the right - we can easily imagine two people, one in left 2nd std dev, one in the right. THey both get happier as they get older, but the person on the far left of the bell curve just never gets to be as happy as the other was in their twenties.

I colloquially would say one is a happy person. The other not.