Again, magic explains a lot, especially when coupled with the technorganic virus that was also essential the Necrosha resurrections.

If the spell decreed "I restore you to life as if you had never been gone" then it might take that literally and age them to match where they should be if they hadn't ever died.

Then again, the resurrected Hellions didn't appear to be any older than they had been, so it could be Doug actually wanted to be the same age as his friends and asked the TO virus to age him a bit when it reconstructed his body from the corpse that he had been.

If in doubt, "A wizard did it", is usually as good an asnwer as you'll get... if it's good enough for Xena.