Tara--
In the ballroom world, I'd say that 21 is still young! As the sport is changing in the US, junior level and youth competition is playing a more important role, but it is not the current American standard for pros to have been in training since youth.
So, what's next? Well, there seem to be two reliable routes to pro competition: First, if you have the money, you can find a reputable pro partner/instructor and work up through the ranks of pro-am competition. This is a great way to go if you can, your partner/instructor will have the experience and contacts that you don't. It also gives you the opportunity to gain comp. experience and "get seen". Downside is the expense of many lessons as well as traveling. Also, no matter what, some day you will have to make the transition to teaching and being a pro because you will need the 24 hour dance immersion if you realy want to be a competitor.
So, the second route is, become a teacher, get some training, get some students who want to travel and compete, build a name among other pros as being reputable and stable, then find a pro partner and start investing the hours and hours of effort it takes to be in pro competition.
Either way it is a career that takes determination and a willingness to sacrifice, but it is incredibly rewarding to those who love it. Even if you don't ever become a pro competitor, the friends you make dancing are a special breed and the relationships you form with students over the years are special too.

Good luck to you in competing, but remember that you love to dance and you don't have to be a national champ to be a wonderful instructor