I think, since, in TT, prime years are often 22-26, and since TT is a sport that is much more a sport about precision and technical subtleties and, probably, the technical aspects are more important than in any other sport I can think of (maybe soccer [futbol] as well), I don't know if there are any top players who started after they turned 18.
I know some players who started at 14 with a background in Tennis who got to decently high levels, but not to top 300 in the world.
I think, especially with the advantage the Chinese pros have where, by the age of 12 they have 6+ years and hundreds of thousands of hours training technique. So, if someone was to try and start at 18 and hope to get to the top levels, they would be playing against opponents who had 12+ years of high level training and coaching to compete against.
When you add to that the skills at reading spin, the precision of how you touch the ball when you loop or push (tangential contact touching only 1-2mm of the edge of the ball), and the skills of reading the trajectory of a ball that curves, arcs and kicks more than in any other racket sport, the skills of tracking and intercepting a ball with that kind of arc, curve and kick, the skills you would need to develop to be a top pro in TT are simply different, and and a different kind of "technical," than for a sport like boxing.
This is not to say you don't need great skills to be a boxer. You do. They are just different skills. And those skills usually are trained from a decent amount earlier than 18 too. But someone with amazing talent and power, could get up to speed fast with something like boxing.
But without the training, the stroke mechanics, the ability to track and intercept the ball, the deep understanding of the nuances of all varieties of spin variations, reading deceptive serves, how to counterloop, I am not sure someone at 18 could develop the skills to get into the top 300.
Of course, I could be wrong. But I don't think it has been done. And this has so much to do with how much of a precision sport TT is. It is also why the top women are closer in level to the top men than in most sports. And it is also why certain players like He Zhe Wen can play into their 50s and still be competitive with players in their prime.