Happy Birthday! The title of your blog caught my eye so I had to check it. Trust me you are not even close to being old yet;)
For me getting old started when I had to work full time and support myself. Sponge of your parents as long as you can
I would say I started feeling old when I was in my late 20's. I'm
gonna be 40 next month and it sucks

I have discovered that the way I was thinking made me feel even older. My advise is to not think about getting old or being old. Just live life to the fullest within your means and you won't feel so old.
Just an example of the way me and my friends use to think about age when we were in our teens. If we met someone in there 20's they were old, if they were in there 30's they were dead, 40+ they no longer exist.
The things I don't like about getting older: you develop a history(couldn't do that at age 10 cause you can't remember the 1st 4 or 5 years). Then there's the aches and pains. The big one is you can't be young again and you can't be young again knowing what you know now. Things could be so different if I could.....
Overall getting older is now as bad as you might think if you don't let it get to you. That in itself will put you in an early grave.
So what kind of guitar are you getting?
Take Care,
PR2