To be a professional, you'll need to be playing football, in my opinion, from the age of 5 or 6 and if you have the talent, eventually, a scout will spot you and then depending who he's working for, perhaps a professional or semi-pro club will take you on as a player when you're 16, and you develop and if you work hard in their academy, then maybe the manager will pick you for the reserve team then eventually the first team.
People have criticsed the way that the academy youth systems work, but if you watch MUTV, you can see the academy and reserve players go up through the ranks, and make it.
But if I was spotted and had the chance to play for a big club like United, I'd be happy to play for them all my footballing life and I wouldn't be demanding 100's of thousands of pounds because at the end of the day, you start to think you're better than everyone else, working class standard and you become soft in football, you'd be afraid to get injured etc.
You play football because its a sport you feel passionate about.
Anyway, in football, you can face alot of disappointments because the chances of you getting a professional career in football are incredibly slim, and people like Alan Smith (if you watched his tv documentary a few years ago) nearly didn't make it because the way the system works is a bit like Pop Idol or X Factor.
But if you really really want to be a footballer then keep pursing your dream, just like David Beckham did, he said that ever since he was a kid, all he wanted to do was play football and he made it, so depending on how much you want something, your chances increase.
But I wouldn't put your hopes up high, it's a very difficult industry to get into, it's a very competitive industry because there are alot more people who want the same thing.