Well this is what I will say and somebody else would probably have a different answer. Taking any martial art is always good. If what you are looking for is participation in tournaments your best bet is to look for an instructor who concentrates on that subject. A lot of martial arts schools go to tournaments, but this does not mean that tournaments are their main dish. It is just part of their curriculum of activities. Now there may be schools that only concentrate on tournaments too. Martial arts are more than just going to a tournament and look good in front of a judge. It is a matter of opinion, taste and/or appreciation of a person on a specific day. This may not happen on subsequent tournaments due to a number of reasons (the judge may be different, they do not like your style, etc).
Tournaments help you in a lot of ways, but also give you a lot of bad habits that you main instructor has to clean up. Over all they are a good experience, you meet a lot of interesting people from all kind of styles, systems or whatever they want to call themselves. Friendship is the best thing that you can get out of these activities. Trophies become old and dusty, friendship last for a long time.
With all this said and to answer your question, no, you are not too old. There is not age to compete in tournaments. You will need a good instructor that like and do them as the main or one of the main activities in his/hers school. As for you last question about how long, well it all depends on how much time you put in your training to progress to a competition level. One more time this is according to your instructor.
I could say more but I think this kind of cover your questions...