The arguments about charities not wanting to be swamped by the unskilled are fair points, but at the same time, charities still do need unskilled hands. They just don't need crazy amounts of them. The article should have been more about the imbalance in skill requirements, not that charities basically only want experts in the field. And even then, you don't need to be a ten-year expert to be skilled enough for a lot of the things a charity work requires. One week's pulling cable would be enough experience to really help out with ghshephard's example above. Sure, a professional netadmin would be better, but a few days cable-jerking would have made someone baseline useful. Likewise team management skills can be very handy in managing the unskilled hordes, and it doesn't take anything like 10 years to gain those kinds of skills.
Taking the article on its merits, no-one would be able to volunteer until their late 20s, which is clearly an absurd concept.