Kids are almost always taught the "proper way" to play by coaches. But older players sometimes already have bad habits or have developed an unconventional playing style. Should they also be taught the "proper way" to play?
The answer is . . . it depends. Some older players may simply want to play like the top players. An older player who is a good physical athlete might also want to learn to play like the top players, i.e. the "proper way," since they have the athleticism to do so, even if they are learning it late. In general, if you want to become a good player, it's usually best to learn the "proper way," and focus on learning the tried and true techniques that are used by top players.
And yet . . . this isn't true for everyone. If you've spent years trying to develop a strong forehand and failed, but have developed a really good backhand and a nice blocking game, perhaps you should forget much of the "proper way" and focus on dominating with your backhand and blocking game. This doesn't mean you should stop trying to develop your forehand - the better it gets, the better you'll be - but perhaps, at this point, it's best to focus on having a forehand that doesn't lose for you, and then you can win with your backhand and blocking. Or pushing. Or whatever else you actually do well.
Similarly, if there's a technical flaw in one of your strokes, but you've done this shot that way so long that it's ingrained, it might be better to just go with that shot, and learn the rest of your game the "proper way."
There's nothing wrong with having something unique in your game that isn't the "proper way" - in fact, it'll win you matches since opponents aren't used to it. However, it's best not to go overboard. Surround these unorthodox strokes or tactics with an otherwise solid game i.e. the "proper way" - and you may reach a higher level than if you just mindlessly try to play the way the top players play.