Hi Shydancer
First of all, at 25 you're not too old. In fact, I don't believe you're really ever too old (maybe not just as flexible,quick, agile or whatever) to attempt most things.
If you want to do it try it. It's better to have made the attempt and failed/succeeded than to look back and say you wish you had tried it.
There are a number of 'older' students at the studio I attend who are competing in Dancesport and quite a number of them are either late 30s or in their 40s. I also recall a lovely couple of Rock and Rollers who were both in their late 40s and still competed successfully. I met them in a club and we got to talking. They were wonderful people and marvels on the dance floor. In fact, I recall an occassion where the husband was throwing his wife into the air and performing all sorts of amazing steps. They made my jive look like an infants first attempts at walking.
As for starting at a younger age I would encourage my children (when I actually have some) to try it if they wanted to. However, as with sports, I would want to ensure they received good guidance. What I mean by that is that during a Dancesport competition I watched an 11 year old boy throw a tantrum and proclaim he would never dance with his partner again. This was apparently because "she was hopeless and made him look bad". This behaviour really annoyed me; even more so when his parents idly stood by, not appearing to be embarrassed by his yelling and screaming.
I think, irrespective of age, if you have the desire and willingness then there's no harm in the attempt.
Go out there, Shydancer, and show them what you're made of!!!!
(insert clapping and cheering here...)