If you think about it, although the Pokemon world obviously revolves around Pokemon (from what we can see), training itself isn't exactly a practical way of living. Yes, you get to travel around a lot, but your only pay is apparently from your winnings (or the League, depending on which canon you're working with and whether or not you're a Gym Leader/otherwise associated with the League). It's essentially a sport or past-time. One that you have to be involved in on a pretty hardcore level, but still.
As in, I have no doubt that a lot of kid trainers simply give up training and opt for taking on something more practical, like becoming a doctor or something. So, you've got kids, teens, the possibly retired elderly, and those working for the League who have the option to spend all of their time on training and people who might still train Pokemon but are primarily something else in order to pay the bills and put food on the table.
*shrug* Really, it's just a theory, so I really couldn't say.
But I will say that I've always thought Steven (and, for that matter, Wallace) wasn't a teenager but was, rather, in his early to mid twenties. Then, if you count Gym Leaders, you've also got Surge, Koga (who's also an E4 member in GSC), Chuck, Norman, Byron, and Crasher Wake. Danny and Luana in the Orange Islands in anime canon as well. On the E4 side, you've got Bruno, Karen (who strikes me as being the same age as Cynthia), Lucian, possibly Sidney (hard to tell his age), and Glacia (same). Maybe even Lance, if you think he's roughly the same age as the other champions. Of course, that depends on whether or not you think any of them are good. From an in-canon perspective (as in, if we try to see it from the viewpoint of someone in the Pokemon world), I'd assume they would be if they got the positions they have.
That's, of course, not counting the older teens anyway. Or, for that matter, the leaders of the evil teams (Giovanni, Maxie, and Archie included, along with the members of Snagem and Cipher.) But in any case, I wouldn't say there's only one or two.