Piece
Tour Winner
My assumption is that they do this to ensure that people don't come along just because it is Sunningdale and they want to play, despite having no golfing ability. They may slow the pace of play down or at extreme levels, damage the course. 18, 20 - doesn't matter I don't think. They just picked the number to stop non-golfers turning up on corporate gigs I would think.
Not that I am saying that everyone would do this...., nor that 18+ = non-golfer.
Spot-on and this clarifies the OP. Open courses that I've played do exactly the same thing. They also demand a current handicap certificate - I've never had one in 30 years and always managed to get on these type of courses. They'll look at your swing and etiquette, and if its acceptable then you play.