I had it at 4, I had 6 spots.
My father had it at 6, 10 & 12.
My sister had never had it yet never caught it.
It is possible to pick up a mild internal case where you actually display no symptoms yet develop immunity.
Having it as an adult can be very severe a lot of adults end up hospitalised.
I know 3 people who have had shingles but no visible Pox (I would presume internal only)
I hope that helps clear up some confusion for those who have had it twice/developed immunity without having it. i did actually once ask my doctor and he agreed that yes these are all possible.