As I understand it, clubs are permitted to offer incentives for groups underpresented. So suspect this would hold up to legal challenge.
On the one hand I can see why those paying full whack would be miffed - its not often the case that people in those age ranges are earning significantly less than those older any more so why should they get a discount.
However, on the other hand it becomes more of a question of who gets the most out of a membership. A person up to the age of 35 might be cash rich but time poor - juggling full time jobs with a young family. As such they might get out at a weekend and possibly one evening a week at best, and even then only for 4 month period of the year. Whereas those with older/adult children are more likely to get more value from their membership cost.
For me personally, as a 30 year old with a young daughter, I don't think I could justify the non-discounted cost of the membership even at our relatively modest club. As it stands at the moment though, it works out about 42 quid a month and is similar to a gym membership for me which is much more palatable.
From the clubs perspective, they have to strike a balance between losing their younger members and having an aging membership with a threat of obsolescence (as my club were close to experiencing not all that long ago), or keeping the younger members with the hope that they remain members for the next couple of decades.
Obviously if you're a club with a a waiting list then you can clearly afford to do away with discounted membership fees as the demand is there.