Is it true the younger the happier, the older the sadder?

I'm 25, turning 26 soon.

I felt like I was still a kid yesterday, and now suddenly I'm all grown up and getting old.
This feeling really makes me feel sad when I realized that I'm no longer young, and would only get older and older.

Time used to fly so slowly in the past, a day used to be like a year for me, it was like living in eternalty. Now I feel like time flies so fast that I'm turning 30, 40 soon, and that I barely have done anything. And what I realize is, the older I become, the harder it is to make friends with other people, so now I'm pretty much friendless. I also found that lots of feelings, such as smell, taste, emotions, memories, hopes are starting to fade away, the world seems to be different.

So is it true that the older you are, the sadder you are? Is it the same for everyone or is it just for me?