British stamps issued since decilalisation on 15th February 1971 are staill valid and I occasionally see forty year old ones.
The decimal half penny was withdrawn in December 1984, 12½p second class and 20½p overseas rate being the last such ½p stamps issued in the preceding year.
Stamp collecting was at its height around 1990 so regularly used are stamps from around then that were bought by those wrongly thinking there would be a demand for them in the future.
I've seen Belgian and USA stamps of the 1950s used a couple of years ago !