I opened my first business last year when I was 21 years old. Before that I had worked for a few small business's and felt comfortable with the fact that owning a business is a lot of work. Many young entrepreneurs seem to think owning a business is 99% doing the services that you offer when in reality it is more like 50% client work and 50% administration.
I did not realize how hard it is to attract clients and get paid for the work you do!
Tools that make my life easier:
My first business was ATP Productions, a one-man (well, technically, kid) shop that built a pretty cool (as I remember it!) shell for MSDOS-based 286-era computers. It had all sorts of cool features like encrypting the FAT so that even if someone booted from a floppy, your files were safe (BIOS things like boot order and passwords were not common in those days). I think it even had a "program installer" that would copy files from floppy disks and automatically install them on the menus, etc.
I don't even remember how old I was... 13... 15 maybe?
My friend (kinda funny to think about now) also had a software company and built the stupidest little program: a boot protector that sat in the MBR. That's it. It had custom ANSI screens and, as a shareware limitation, showed the password in plain text. He sold like $10K-$15K worth of licenses.
I think I sold maybe one copy of mine, or maybe it was a beta installation... I don't remember. But the big learning lesson was that the program was way too complicated for the shareware BBS distribution market. It would have been perfect for a school or other environment where they needed to maintain a network of computers with common configuration.
Not counting the lemonade stands I used to setup as a pre-teen (on my very untrafficked street, no less...), I started a lawn service when I was 17. That service employed 3 people over the years, and netted me enough to cover all non-tuition expenses at my college. I eventually sold it to my father who retired and wanted something to do and keep him healthy.
When I was 11 I had several "businesses", which included pool cleaning, baby sitting and tennis ball collecting (selling the balls to people that used the local courts). I had "employees" in all of these businesses, and I shared the profits with them.
At 15 I started fixing computers, and at 17 I applied for a patent for a credit card that kids can use (despite the fact the cannot legally sign for deals, since the parents authorized the deals ahead of time). That was a very expensive lesson (for my father) that you need to know your customers before you start spending money on legal issues :)
I remember collecting old newspapers and selling them for recycling at around 10. I was also running a gardening business at the time.
I'm happy to say that at 46 years old, I've always worked for my own businesses and never had a "proper job" ;-)
The earliest recollection of "doing business" was when I was 10. I started to sell pokemon game with emulator included on a floppy disk. Then my parents caught me and scared me into not selling it any further. I continued by writing down the url to download the pokemon rom and emulator and sold this piece of paper for a dollar, and made sure that they read the "Terms of Agreement" I copied from the same URL. I made a dollar and then kids were complaining this was fraud. Then I tried to calmly explain that you need to pay for information you don't have the ability to acquire. Then they threatened to rat on me to the teacher and I knew it was time to shut it down.
I think the first "business" I started was in second year of university, at 21, running my own shopping site where people could purchase php scripts I wrote while being bored in between lectures. I was shocked to sell over 2 grand worth and I think this is what got me to doing what I am doing today at 24.
I started when I was 23 and I'm 26 now. Went from nothing to a 7,000 sq ft studio and about $500k in equipment as a creative agency. Bootstrapped the whole way through. No loans or debt. I took on clients and started raising my rates every time my work got better. I also found very good people to work with.