This is a good question. 10: First entrepreneurial effort selling lemonade. 13: First software entrepreneurism selling a game on floppy disks. 16: First founded a company, ostensibly to make games. 18: First made a living contracting for myself. 25: First founded a company that became profitable and hired people. Which one counts?
So the web design gigs at 15 and forming an LLC for part-time consulting don't count.
I came up with this definition after reading a book where the author bragged he'd started 10 companies by age 21... by the above definition the number was actually 2 -- still impressive.