It depends on how they're growing. If they're solid and a good size, I start the switch at 6 months to avoid too much weight gain putting stress on those developing bones. If they're small and need more substance, I'd stay with puppy food until 8-9 months.
Most good quality foods have nearly as much calcium as puppy food does so even on adult food the puppy will get enough calcium. Puppies need more protein but again, good adult foods only have slightly less protein than comparable puppy foods. It's important to read the labels so you can compare before making the switch.