I can belt out some noise to match any young gun... I just can't leap up and down any more!

Ok I think encourage might be a better word than mould.
And those fans standing with their backs to the match for large portions of the game are rallying the rest of the fans who in term have been the 12th man we've so badly needed for the last few seasons and to who we could give some credit for keeping and getting us up.
To be frank, the more fans we have up and singing the more intimidating it is for the opposition and not to mention more enjoyable the game is for us due to the atmosphere.
If some of the older fans want to sit there and watch the game quietly that's fine, but I think what the OP was getting at is to get the new and younger fans to be part of the loud bunch which can only be a good thing for everyone.
So yes, let's ENCOURAGE as many people as we can to get involved, sing up and make selhurst a party for us and absolute hell for any visitors.

We can all talk about which is the better supporter etc. The reality is nobody is. If you compare football supporters to customers of a business (in many peoples eyes that is exactly what they are) and say which customer would you prefer? The one that's been buying from you for a couple of years and raves about your product so much so that he/she has begun to encourage their friends to buy from you too. A very valuable customer you'd think and you'd be right but what about the loyal customer that does not make any kind of a fuss at all but buys from you every week, has done for 50 years and because of his/her actions their family have grown up loyal to your business and buy from you too. But lets say they also do not make a song and dance about your product, they just buy it.
The reality is neither supporter is better than the other and both are just as vital to the long standing survival of the club.
We should learn lessons from other football clubs. If we discourage singing/atmosphere Selhurst park will be like Arsenal without the quality football. People will start dozing off. On the flip side, if you discourage/ostracise quiet fans in favour of singing/chanting/jumping around you risk ending up doing what Millwall did in the late eighties and drive more than half your support away. They still haven't got those fans back even today. Partly due to the fact that they're crap but there you go, I think you see my point.
In short, Palace is fine just the way it is. In my view, ALL fans welcome whether you are 5 or 85, a passionate singer or a subdued little wall flower and everyone in the middle!

Just to make sure my original point is clear. I entirely agree that it is fine not to sing if you don't want, especially if you are a long-standing fan that contributes in other ways and has for many years. We are a broad Church.

I don't think it's correct to assume that as you get older you automatically don't want to sing but perhaps it is true that the most enthusiastic singers are not yet retired.

My point, as above, was always don't knock the new supporters. Rather than all being plastics who will move on if we stop being successful, maybe instead they are on a journey to learn how to be outstanding supporters. They are learning in the best possible place. We just need to hold onto them for long enough so that, using my earlier word, they get moulded. And that won't be anybody actively moulding them, they will just have served their apprenticeship. Poor souls, once hooked there will be no turning back.....