I think it all comes down to having realistic expectations...one is never "too old" to start playing for fun, but if your goal is to go to conservatory, then you should probably start when you're a little kid. everyone talks about how much faster kids learn, which is certainly true for the kids that "get it," but what about the great majority of kids who don't make progress, don't practice, and drop out of lessons? the fact is that most people who try anything hard don't fully succeed, but that doesn't mean it's not worth doing.
in your initial post you said you felt your learning had reached a plateau. We all reach them periodically. the key is to change your approach and instead of trying to keep going in the same direction, look in a new direction. so if you've got a piece that you're just not getting anywhere on, set it aside for a few days/weeks/months and try a new one, or a new practice routine, or something. when you return, you will be rusty at first but then in general be amazed at how much progress you made without even realizing it.
one thing that I do a lot is listen to music, even exceptionally difficult music, and try to follow along with the score. as a result I have a stack of scores that I'll probably never be able to play, but it helps me to learn, especially in terms of sight reading.
by the way, I'm 30, and while I dabbled in various instruments as a kid, I didn't seriously take up the piano until 2 yrs ago.
oh yeah...with any musical instrument it takes at least a year of serious study to really get much "real music" to come out, and even the great masters are never happy with their playing!
best of luck!