I think perhaps the thing they're actually not picking up on is that Charles was The Prince of Wales and the heir to the throne already when he was William's age.
The royal family was also smaller then, I think -- in 1974, when Charles was 26, the working members would have just been The Queen, Prince Philip, Princess Margaret, the Queen Mum, Prince Charles, Princess Anne, the brand-new Duke and Duchess of Gloucester, Princess Alice, the Duke and Duchess of Kent, and Princess Alexandra, right? Today we've lost Margaret, Alice, and the Queen Mum, but we've gained The Duke of York, The Earl and Countess of Wessex, The Duchess of Cornwall, Prince William, and Prince Harry. More people to spread the work around to (or, I guess it could be argued, more charitable opportunities to be taken up).
All that said, I do think it would be nice if we saw William out conducting more official royal business. I don't know what he's waiting for. He could be doing enormous good for lots of people just by attaching his name to more causes and standing in front of photographers at more charitable events.