Scared of getting old

Since high school I always wanted to grow up and achieve all these things i was working towards, but now that i have almost everything i have strived for i wish i could take back the time. I now feel old, I now feel the pressure to settle and get my life together. That’s scares me. Moving through my twenties I realised how much my life have changed in a such small period of time. My social circle has shrunken, people that we used to share everything now feel more like acquaintances and i was hit with the realisation that friendships are no ones priority anymore. If you think about it, it makes so much sense. Then why i have such a hard time accepting it? Why am I so scared of having fewer good friends than i used to? Is it reasonable to think that twenties is the time you need to get my life together? Why do I still feel like a child and still reminisce my life in high school as i now have everything i wished for back then?