I believe there is a requirement of doing one full season before applying for promotion (could be wrong though). I wouldn't be too hasty about wanting to rise the ranks. It's better to gain the experience and consolidate yourself at each level I think.
I've just started OA this season at the age of 20 and after 4 seasons doing youth games. I would say go for it. Get in touch with your local league, explain your situation and they'll put you on a game. Usually the refs secretary is experienced, knows the teams and will give you nice games to start with.
Your confidence will grow as time goes on, it's normal to be apprehensive about controlling 22 fully grown blokes (especially if you're only 5'6'' like me). The main thing I've found is just talking to them helps no end, have a chat before the game, be proactive before a challenge happens (e.g. "go easy in the back number 6", at corners "no holding as we run in please lads", throw-ins "hands down as we jump please") they've then got no defence when you pull them up for it.
Today I had players genuinely surprised that I turned up 30 minutes before ko to check the pitch and that I did a warm up. Little things like that get the players slightly on board at least before you've even started and show you actually want to be there.
In the 6 men's games I've had so far this season I've found that the some of the player's are a bit mouthy and require a stern word early on, some will question you but if you're honest and explain yourself they'll leave you alone but the overwhelming majority just want to get on and play football with their mates.
Good luck with it!