Old stamps put to use.

I had 2 letters come through the post office today with these stamps on them.
The 6c orange stamp was issued in 1970, when I was only 10 years old, that's giving away my age ;-) and the 18c stamps were issued in 1975.
Someone must be using up some stamps that have been saved for a long time. I can just see my kids one day using the stamps I have been saving for the last 30 odd years.
I must say I got a real kick out of seeing them on the letters today.